As genetics researchers acheive greater success in their attempt to map the exact location and functions of the human genes, the [u]close connection between genes and birth defects has become increasingly clear[/u].
a) close connection between genes and birth defects has become

b) closeness of the connection between genes and birth defects has become

c) close connection genes have with birth defects has been becoming

d) close connection between genes and birth defects has been becoming

e) close connection between genes and birth defects becomes

Could some one explain the correct answer. I will put in the OA later.