Which statement is not true regarding natural vs. synthetic materials

Which of the following is the earliest era of earth's geologic time scale? cenozoic mesozoic precambrian paleozoic

Why did adult crab numbers decline in he ecosystem, even though sea stars were still available?

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.