单项选择题
A mobile offshore drilling unit under tow is approaching a fog bank. When should fog signals be started? （）
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
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A.under tow
B.fixed to the seabed
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D.All of the above -
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A message by flashing light consists of the call，the identity，the text，and the （）.
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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?（）
A.The motion of the apparent sun is along the ecliptic
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