Impressive Was a Champion Quarter Horse Stallion with a Magnificent

Impressive was a champion Quarter Horse stallion with a magnificent muscular physique,a trait that he passed on to dozens of offspring.However,many of Impressive's offspring developed periodic muscular twitching that sometimes left the horses temporarily unable to move.Some horses even died.As it turns out,Impressive's award-winning physique was due to a mutation in a single,protein-encoding gene,resulting in hyperexcitable muscle (which contracts even in the absence of a stimulus from its motor neuron) .Which is NOT a plausible explanation for the identity of the abnormal muscle protein in Impressive and his offspring?

A) acetylcholine receptors that function in the absence of acetylcholine
B) leaky Na⁺ channels
C) overactive Ca²⁺ pumps in the membrane of the sarcoplasmic reticulum
D) tropomyosin that is unable to bind to actin
E) over-active troponin molecules