The river level was rising drastically after that heavy rain.

A. slightly

B. suddenly

C. greatly

D. unexpectedly

Answer: Option C

Solution(By Examveda Team)

Slightly : to a small degree; not considerably.
Suddenly : quickly and unexpectedly.
Greatly : by a considerable amount; very much.
Unexpectedly : in a way that was not expected or regarded as likely.
Drastically : extremely; very.

Synonym of Drastically is Greatly