Which of the following best describes how crossing over occurs during meiosis?
a. during prophase i, homologous chromosomes are lined up and pulled apart to opposite poles.
b. during prophase i, segments of chromatids are exchanged between homologous chromosomes.
c. during prophase ii, chromosomes overlap and sections of each chromosome switch places.
d. during prophase ii, sister chromatids trade places with each other before metaphase begins.