This represents which limit of natural selection

a. natural selection can only act on existing variation

b. adaptations are usually compromises

c. natural selection interacts with chance and random changes in environment

d. natural selection is limited by historical constraints

a. natural selection can only act on existing variation

Explanation:

Natural selection is the process of differential survival and reproduction. It occurs when a beneficial trait it passed on from the parents to the offsprings in the population of the same species. A beneficial trait will not be passed to the offspring if the trait is non heritable. The trait will be passed on as a genetic component. For example, beak length the trait for beak length or genetic variation is already present in the population. It is selected on the basis of the natural selection and can be expressed in the new generation. The natural selection process acts on the individual but the results of the selection can be seen on the entire population.              

after a the power stroke, adp is released, but the cross-bridge formed is still in place. atp then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.

when it is at an intermediate population density