What is needed for the myosin head to attach to the binding site on the actin molecule?
a. atp
b. sarcomere
c. calcium
d. sodium

The correct answer will be option-C.

Explanation:

The muscle contraction occurs as a result of the sliding of the myofilaments made up of thin actin filaments and thick myosin filaments. The actin contains a binding site for the myosin which is covered with the tropomyosin-troponin complex proteins.  

The troponin contains a binding site for the "calcium" which is released from the sarcoplasmic reticulum in response to the nerve signals. The calcium binds the troponin and causes conformation changes in tropomyosin which exposes the myosin-binding sites to bind myosin. The myosin binds and forms cross-bridge and cause muscle contraction.

Thus, Option-C is the correct answer.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."

cell division is not regulated in tumor cells.