The water-holding capacity of soil is LEAST likely to be affected by the addition of which of the following?

clay

humus

manure

pesticide

sand

During binary fission, what events must occur before the cell splits into two new cells?

What factors contribute to the effect an environmental toxin has on the human body?

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.