单项选择题
The radiotelegraph alarm signal is（）.
A.SOS sent three times B.ten one-second dashes with a four-second interval between them C.twelve four-second dashes with a one-second interval between them D.twelve one-second dashes with a short interval between them
单项选择题 The expression FIRST MAGNITUDE is usually used to refer only to bodies of magnitude（）.
单项选择题 The expression DROP ANCHOR means（）.
单项选择题 The best method to stop a vessel from dragging anchor in a sand bottom is to（）.
