The bilge system has been performing well; however, the aft starboard engine room bilgewell suddenly fails to be pumped outWhich of the following should be done first to determine the cause?（）

A. Open the bilge pump for inspection
B. Remove each of the manifold valves
C. Remove only the manifold valve to the affected bilge-well
D. Attempt to pump out another bilge well to determine if the entire system is affected