单项选择题 We took usual steps:sounding the depth, reversing and stopping engine,paying out anchor and 60 fathoms of chain cable in the depth of 12 fathoms.The operation above is（）.

单项选择题 Which of the following is not a factor which can interfere with the pulse of sound and cause a variation the depth reading?（）

单项选择题 Who can contact the Designed Person?（）