The smallest particle in the universe? A grain of salt is small, but you can always make it smaller. Imagine cutting that grain of salt into two pieces. Now cut it again and again. Soon, you can't see the smaller pieces with your eyes, but the salt is still there. You finally cut the salt down to the very tiniest piece of salt there is. But even that tiny piece contains smaller particles. Those tiny particles are atoms. Atoms make up everything in the visible universe from galaxies to even yourself. Atoms are so incredibly small that you could line up 50 million in a row and the line would only be about 1 centimeter (less than half an inch) long. Still, scientists have found things that are smaller than atoms. And they are looking for more. If they find the smallest things in the universe, they'll better understand how the universe actually works. But it took some time before people discovered the world of the truly small. The Universe Gets Smaller… Grains of sand or dust were once the smallest things actually seen on Earth. By the 1600s, several inventions opened up brand new worlds to curious minds. These included lenses that could make things look clearer and bigger. Another early invention was the microscope. Some people used the microscope to observe and write about the tiniest things they could see. In the 1670s, a Dutch lens maker named Antonie van Leeuwenhoek built himself a microscope. It magnified things more than 200 times. Van Leeuwenhoek discovered a world of tiny living things that he called tiny animals. Van Leeuwenhoek figured they were about 1/38th the size of a grain of sand. Today we know that what he saw were bacteria, the smallest living things on Earth. But atoms are much, much smaller. You can't see atoms with an ordinary microscope.
…And Smaller The idea that tiny, unbreakable particles make up everything that exists is more than 2000 years old. The Greek thinker Democritus called these particles "atomos." This is the Greek word for "uncuttable." Scientists didn't return to the idea of atoms until the 1800s. At first, scientists thought atoms were tiny balls with some electrical charges inside. They also thought atoms were the smallest particles that existed. But scientists soon began to wonder if atoms might be made of smaller things. In 1897, British scientist J. J. Thomson proved that they were. He ran experiments and discovered the electron. This tiny particle has a negative electrical charge and whizzes around inside the atom. A graphic showing the basic atomic structure of three elements, hydrogen, helium and oxygen. Protons, neutrons and electrons are shown. Zoom-in Different elements have different numbers of protons, neutrons, and electrons. The Smallest Things—So Far Scientists were soon discovering more inside the atom. Hiding in the atom's center is the tiny nucleus. (If an atom were the size of a racetrack, the nucleus would be about the size of a pea in the middle.) The nucleus contains two types of particles: protons and neutrons. Protons have a positive electrical charge while neutrons have no charge. They contain even tinier particles called quarks that are so unimaginably small that they have no internal structure. Quarks and electrons are the smallest particles found so far. Scientists call the smallest things they've found fundamental particles. Fundamental particles do not contain any smaller particles. Scientists use huge machines called particle accelerators to learn more about particles. These machines speed up particles so they can smash into each other. Then the scientists track the paths the particles leave when they hit. Scientists use accelerators to discover new particles. Many scientists wonder why there are so many particles at all. Shouldn't there be just one "smallest thing" instead of many? The search goes on for the particle that is the one true building block of everything in the universe

Which sentence from the article helps the reader to understand that the nucleus of an atom is very small?
A Still, scientists have found things that are smaller than atoms
B The idea that tiny, unbreakable particles make up everything that exists is more than 2,000 years old
C Hiding in the atom's center is the tiny nucleus
D (If an atom were the size of a racetrack, the nucleus would be about the size of a pea in the middle.)
Read the paragraph below from the section "... And Smaller."
But scientists soon began to wonder if atoms might be made of smaller things. In 1897, British scientist J. J. Thomson proved that they were. He ran experiments and discovered the electron. This tiny particle has a negative electrical charge and whizzes around inside the atom
What inference can the reader make based on this paragraph?
A Electrons are the made up of even smaller particles
B Other scientists had questions about Thomson’s experiments
C Thomson could not see electrons but found signs they existed
D Thomson discovered electrons while looking through a microscope