A （）, sometimes called filters, is a device designed to prevent the passage of unwanted solids into the system.

A.a check valve
B.a gate valve
C.a pressure reducing valve
D.a quick closing valve

A.at most references
B.at least references
C.maximum standards
D.minimum standards

A.pollution by oil
B.pollution by noxious liquid substances carried in bulk
C.pollution by sewage from ships
D.pollution by garbage from ships