Some adaptations, on the other hand, become useless. These adaptations are vestigial: remaining but functionless. Whales and dolphins have vestigial leg bones, the remains of an adaptation (legs) that their ancestors used to walk. Why did the author include this selection?

This is because vestigial parts are usually homologous to most of the structures that are functional in other species of organisms. E.g.. the Vestigial legs which are non functional in  whale and Dolphins, are useful to other organisms. Hence, if these organisms possessed these structures, they should share certain characteristics with  each other.Therefore these organisms can be said to evolved from one another.Thus their vestigial organism should be included by the author  as evidence of evolution.

Explanation:

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

read more on -