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單項(xiàng)選擇題
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
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