In modern fathometers the sonic or ultrasonic sound waves are produced electrically by means of a(n)（）.

A.Effect of the moon's gravity
B.Fetch
C.Wind velocity
D.Length of time a wind has been blowing

A.ahead of the storm center
B.in the dangerous semicircle
C.in the navigable semicircle
D.directly in the approach path of the storm

A.Alaska Current
B.Subarctic Current
C.Kuroshio Current
D.Oyashio Current