初期坝的坝型可分为（）

A、不透水初期坝

B、不透水初期坝、半透水初期坝

C、不透水初期坝、透水初期坝

You should conduct a sector search under which of the following circumstances（）.

A.The search target is sighted and then lost

B.More than one vessel is available for a search

C.The search object is a target that will be readily detected by radar

D.An aircraft is available to assist a surface vessel