Which statement correctly describes the relationship between bones and muscles

a. a muscle can both push and pull a muscle, causing it to move
b. bones can move without the aid of muscles
c. moving a bone back and forth requires the action of at least two muscles
d. a bone needs only one muscle to move it back and forth

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.