Based on the Classic Experiments of Creighton and McClintock with Maize,crossing

Based on the classic experiments of Creighton and McClintock with maize,crossing over involves a physical exchange between chromatids.What particular chromosomal characteristic allowed their experiments to succeed?

A) colored bands along the lengths of a chromosome
B) an extra chromosome
C) a missing chromosome
D) three chromosomes with identical structure
E) a chromosome with a unique cytological marker