How were the rough locations of genes and chromosomal processes such as crossing over first visualized in detail?

A) by X-ray mutagenesis of Neurospora chromosomes
B) by isolating chromosomes from the pea plants used by Mendel
C) using unusual, abnormally large chromosomes from Drosophila
D) using X-ray crystallography to examine chromosome structure
E) by examining mutations in sperm and egg chromosomes