How could crossing over affect the distribution of the four alleles: A, a, B, and b?

A. Crossing over would always keep A and B together on the same chromosome, and keep a and b together on the same chromosome.
B. Crossing over would always put A and B on different chromosomes, and always put a and b on different chromosomes.
C. Crossing over would sometimes, but not always, exchange the positions of A and a, and of B and b, on homologous chromosomes.
D. Crossing over would sometimes, but not always, delete one or more of the four alleles from both homologous chromosomes.