Which of the following correctly differentiates the types of muscles based on the type of movement that they produce?

a: smooth is voluntary; skeletal and cardiac are involuntary
b: skeletal is voluntary; smooth and cardiac are involuntary
c: cardiac is voluntary; skeletal and smooth are involuntary
d: smooth, skeletal, and cardiac are all involuntary

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.