In meiosis,crossing over is initiated at:

In which of the following,each chromosome is seen to consist of a single chromatid?

Crossing over is the exchange of genetic material between

$A$ : Anaphase-$I$ is the actual phase of reduction in the number of chromosomes.
$R$ : Homologous chromosomes move to the opposite poles with both their chromatids.

Bivalent chromosomes align themselves at the equator during

In which phase of meiosis do the sister chromatids separate and move towards opposite poles?