What causes the differences in physical characteristics like hair color among people? different versions of genes
variations in DNA length
different numbers of chromosomes
variations in cell nucleus size

different variations of genes

Explanation:

DNA length has nothing to do with differences in physical characteristics. Chromosomes control hair color but the number and sizes of chromosomes doesn't play any sort of factor. Different variations of genes is the only answer that makes sense.

they could find out our specific needs.

answer; being clearly political;