单项选择题
The interval of the average elapsed time from the meridian transit of the moon until the next high tide is called the（）.
A.harmonic constant B.establishment of the port C.half-tide level D.tide cycle
单项选择题 The Humboldt Current flows in which direction?（）
单项选择题 The easiest way to anchor a vessel in a current is to（）.
单项选择题 The difference between the heights of low and high tide is the（）.
