I think the correct answer from the choices listed above is option A. When crossing-over takes place, chromosomes exchange corresponding segments of DNA. Corresponding genes on non-sister chromatids are exchanged at chiasmata. Hope this answers the question.
x or y because both can cause it
B. 25% because Bb x Bb= Bb, Bb, Bb, and bb. The only one that will actually show the red feathers is the completely receive one: bb. This is only 1/4 which equals 25%
The base level of a tributary would become HIGHER if a dam and a reservoir were constructed where it flowed into a trunk stream.