During periods of vigorous physical activity, a person's breathing and heart rates increase. this enables the cells of the body to perform more efficiently because it the cells to

Activate,cells can be classified by their activity.For example fat cells are one of the least active cells and it's hard to activate them;that's why it's hard to lose weight.The least active cells are bone (aka bone marrow cells) that are never active unless there's a fracture.Cells can also be classified by the amount of water they carry and their age.So when you're doing a lot of physical activity your heart rate and breathing increases because your cells are more active.

The right answer is b hydrogen bonds!