单项选择题

A message by flashing light consists of the call，the identity，the text，and the （）.

A.destination
B.answer
C.time
D.ending

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A mean sun is used as the reference for solar time for ...

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?（）

A.The motion of the apparent sun is along the ecliptic
B.Measurement of time is along the celestial equator
C.The speed of the Earth's revolution is not constant
D.There are variations in the Earth's rotational speed

A marine chemist issues gas free certificates and is ce...

单项选择题A marine chemist issues gas free certificates and is certified by which organization? （）

A.Mine Safety Appliance Association
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A major health hazard of the product tert-butylamine is...

单项选择题A major health hazard of the product tert-butylamine is that it （）.

A.can be absorbed through the skin
B.causes irreversible damage to eye tissue
C.is a very unstable product
D.All of the above

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A message by flashing light consists of the call，the identity，the text，and the （）.