The general meaning of corpus means body in Latin. For example Habeas Corpus pretty much means "show me the body".
The fish would die out for the reason of there is no source of food.
The correct answer is A. maintaining homeostasis.
Homeostasis is a term referring to an organisms continuous process of maintaining and auto-regulating the conditions of its internal environment. Variables such as pH, temperature, and fluid balance need to be at optimal conditions in order for the organism to function properly.
In this example, the phosphate buffer system permits the organisms to maintain a constant pH in their intracellular fluid. This is one of the organism's homeostatic mechanisms.
B. Systolic is when the heart beats. Diastolic is the heart at rest.