The ruins of the spillway are a vital clue to the epic struggle that unfold as generations of Khmer engineers coped with a water system that grew complex and unruly.

Options

A. coped with a water system that grew complex and unruly.
B. to the epic struggle that unfold as generations of Khmer engineers
C. The ruins of the spillway are a vital clue
D. No error

Answer

B

Explanation

Here, to the epic struggle that unfolds .... as generations of khmer engineers is the right usage