单项选择题
A towing light（）.
A.flashes at regular intervals of 50-70 flashes per minute B.is yellow in color C.shows an unbroken light over an arc of the horizon of not less than 180°nor more than 225° D.All of the above
单项选择题 A signal of one prolonged，one short，one prolonged，and one short blast，in that order is given by a vessel（）.
单项选择题 A signal of intent must be sounded in international waters by（）.
单项选择题 A sailing vessel shall not impede the safe passage of a（）.
