单项选择题
The term PROLONGED BLAST refers to（）.
A.a blast of from five to seven seconds' duration B.a blast of from five to six seconds' duration C.a blast of from four to six seconds' duration D.a blast of from six to seven seconds' duration
单项选择题 The Rules state that risk of collision shall be deemed to exist（）．
单项选择题 The Rules state that a seaplane shall（）.
单项选择题 The rules require which factor to be taken into account when determining safe speed（）.
