单项选择题
A.When the towing vessel enters the fog
B.When the drilling rig enters the fog
C.When the towing vessel can no longer be seen from the rig
D.Immediately
单项选择题 A mobile offshore drilling unit must display obstruction lights when it is on the waters over the Outer Continental Shelf and is（）.
单项选择题 A message by flashing light consists of the call，the identity，the text，and the （）.
单项选择题 A mean sun is used as the reference for solar time for three reasons. Which reason is NOT a cause for use of a mean sun?（）