After an IOPP Certificate is issued to an inspected vessel，how many other surveys of the vessel’s pollution prevention equipment are conducted during the period of validity of the certificate?（）

A.American Consul
B.Chief and Second Mates
C.Vessel's agent
D.vessel's owners

A.mine clearance operation
B.towing operation
C.dredging or surveying or underwater operation
D.fishing

A.vessel towing a submerged object
B.dredge underway and dredging
C.hydrographic survey vessel at anchor and surveying
D.None of the above