If Na⁺ cannot enter a muscle fiber in response to a stimulus,

A) contraction cannot occur.
B) relaxation cannot occur.
C) Na⁺ will be released from the sarcoplasmic reticulum instead.
D) the active sites are left exposed.
E) the action potential travels into the muscle anyway.

Contraction in muscle fibers is initiated by the influx of sodium ions (Na+) into the muscle cell, which generates an action potential leading to muscle contraction. If Na+ cannot enter the muscle fiber, the action potential cannot be generated, and thus, contraction cannot occur.