When the _______________ _______________ (letter D) contracts, blood is pushed through the aortic semilunar valve into the aorta. 

What theory suggests that the body loses its ability to keep pace with the maintenance of life processes?

a. Genetic mutation theory b. Wear-and-tear theory c. Viral theory d. Environmental theory

Hurricanes are ____ weather systems that form in the ____, resulting in harsh, spinning winds accompanied by thunderstorms that move across the oceans and onto land.

A. high-pressure, tropics B. low-pressure, tropics C. high-pressure, arctic D. low-pressure, arctic