Because populations are genetically open:​

A) ​there are no barriers, cultural or biological, to human reproduction
B) ​there is no such thing as biological variation
C) ​there are few fixed racial groups
D) ​there are no fixed racial groups

Populations are genetically open, meaning that there is gene flow between populations which prevents the formation of fixed, biologically distinct racial groups. This concept highlights the fluidity and interconnectedness of human genetic variation, challenging the notion of rigid racial categories.