A towing vessel 30 meters in length is pushing barges ahead. How many white masthead lights is the vessel REQUIRED to show at night?（）

A.white light
B.red light
C.yellow light
D.blue light

A.special flashing light
B.anchor light
C.sternlight
D.masthead light

A.only if she is towing astern
B.only if the tow exceeds 200 meters
C.at any time when towing
D.if the towing vessel is part of a composite unit