单项选择题 Approaching an anchorage in fog，you hear one short，one prolonged，and one short blast in that sequence on a ship’s whistle． This indicates（）.

单项选择题 An occulting light is one in which（）.

单项选择题 An inconspicuous，partly submerged vessel or object being towed，where the length of tow is 100 meters，shall show（）.