During which phase of meiosis does each of the following events occur?
(a) crossing over,
(b) chromatids separate at their centromeres and migrate to opposite poles,
(c) chromosomes become aligned in pairs at the equator

(a) crossing over: Meiosis I, Recombination  

(b) chromatids separate at their centromeres and migrate to opposite poles: Meiosis II, Anaphase II

(c) chromosomes become aligned in pairs at the equator: Meiosis II, Metaphase II

Explanation:

Homologous recombination is a type of genetic recombination that occurs during meiosis (formation of ovum and sperm cells). The paired chromosomes of the male and female parents are aligned so that similar DNA sequences intersect. This crossing over produces an exchange of genetic material, which is an important cause of the genetic variability observed in the offspring.

Meiosis II: Anaphase II. The centromeres separate and the daughter chromatids - now individual chromosomes - move to the opposite poles of the cell. The centromeres separate, and the two chromatids of each chromosome move toward the opposite poles in the spindle.

Meiosis II: Metaphase II. Chromosomes are accommodated in the equatorial plate of metaphase, similar to what happens in mitosis. They are attached to the already fully formed meiotic spindle. Each chromosome is aligned in the equatorial plate of the metaphase, as it happens in mitosis.

one would be wind another would be water density

colors do exist. the color of the sky is actually violet; humans percieve the color of the sky as blue.