What is the main idea behind the model of a molecular clock?
a. that neutral mutations accumulate at a steady rate
b. that certain traits are under the pressure of natural selection
c. that segments of DNA can be compared with segments of RNA
d. that phenotypes, not genotypes, are affected by natural selection

Johnny rode his bike to a friend's house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was not able to play. what was his displacement for the total bike ride? how did you determine this? what could we use as a reference point to determine he was in motion during his bike ride? why can you use it as a reference point