A person's ability to engage in continued muscular exertion over a period of time,such as carrying a book bag throughout the day,relies on

A) muscular strength.
B) muscular endurance.
C) flexibility.
D) speeD.

Muscular endurance is the ability of a muscle or group of muscles to sustain repeated contractions against a resistance for an extended period of time. This is important for activities that require prolonged periods of exertion such as carrying a book bag throughout the day. Muscular strength, on the other hand, is the ability to generate force against resistance in a single effort. Flexibility and speed may be important for other activities, but are not directly related to the ability to engage in prolonged muscular exertion.