I would say B.
A, umm, don't crash trains, that's bad.
C, Lillian is wondering how fast a train can go, not at what curve will it fall.
D, Lillian isn't wondering about the size of the trains, just the speed.
If it's two different kinds of birds it's called competition.
Answer: space or shelter, ability to reproduce
Explanation:
Crossing over occurs at the four-strand stage when two homologous chromosomes, each consisting of a pair of sister chromatids, are paired. Each crossover involves just two of the four strands and generates two recombinant strands.
Hope this helps!
