Recombination between linked genes comes about for what reason?

A)Mutation on one homologue is different from that on the other homologue.
B)Independent assortment sometimes fails because Mendel had not calculated appropriately.
C)When genes are linked, they always "travel" together at anaphase.
D)Crossovers between these genes result in chromosomal exchange.
E)Nonrecombinant chromosomes break and then re-join with one another.