Mycorrhizae enhance plant nutrition mainly by
a. converting atmospheric nitrogen to ammonia
b. stimulating the development of root hairs
c. absorbing nutrients (and water) through the fungal hyphae
d. providing sugar to root cells, which have no chloroplasts

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.