6. What is a chromosome? a. A small piece of cell membrane
b. A pair of DNA molecules
c. The protein center of a cell
d. Part of the cell's endoplasmic reticulum

The answer is B.

Explanation:

Chromosome is the thread of DNA that is found in nucleus and they are always in a pair.

i think that it is plants

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.