The cytoplasm of animal cells has a concentration of potassium ions (k+) as much as 20 times higher than the concentration outside the cell. how do cells move potassium ions against a concentration gradient, from a lower concentration to a higher one?
a. facilitated diffusion
b. active transport
c. osmosis
d. diffusion

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.