Answer:
B. Different excercises are needed to improve different muscle groups.
Explanation:
Answers A and C are too specific to their indivudal excercises and irrelevant to the question, and D disregards excercises that work out various parts of the body at once. B encompasses what you are trying to answer with the question, explains why different excercises are important, and is general enough to fully adress the broad question.