Which of the following accurately describes nutrient limitation?
a. nitrogen is typically the limiting nutrient in freshwater environments.
b. soil environments aren't affected by nutrient limitation.
c. if one essential nutrient within an ecosystem runs out, primary productivity will be limited.
d. limiting nutrients are the nutrients whose supply is in excess.

During crossing-over, a. genetic material is exchanged between nonsister chromatids, resulting in new combinations of alleles. b. nonsister chromatids from each homologous chromosome of a tetrad are exchanged, resulting in new combinations of alleles. c. one homologous chromosome of a tetrad is exchanged with another tetrad, resulting in new combinations of alleles. d. sister chromatids from each homologous chromosome of a tetrad are exchanged, resulting in new combinations of alleles. e. genetic material is exchanged between sister chromatids, resulting in new combinations of alleles.