HELP PL

Excerpt from Understanding Speed and Sound
Marie Devers

Did you ever think about what sound is made of? Did you know that sounds move at a certain speed? Learning about sonic booms can help you appreciate just how rapidly sound can travel. To understand the term sonic boom, you must first examine some scientific notions. You should realize that everything is composed of molecules. Even air—which may seem like nothing—is made up of molecules. Sound acts like a wave that shoves these molecules around. Some air molecules compress, or move close together, while others spread apart. As molecules move, they force other molecules to relocate. This movement allows sound to travel through the resulting empty space. The speed of sound is the rate at which sound travels through this space.
Which is the topic sentence of paragraph 1?

A) You should realize that everything is composed of molecules.
B) This movement allows sound to travel through the resulting empty space.
C) The speed of sound is the rate at which sound travels through this space.
D) Learning about sonic booms can help you appreciate just how rapidly sound can travel.