What is the main source of downward convection movement in the mantle

I’m stuck on this question cuz i’m stupid sksksksk

Which is the correct formula for photosynthesis

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.