[单项选择]

Everyone uses the expressions AM and PM. But do you exactly know what they mean and how they came into being
As you know, the turning of the earth makes the sun and the stars seem to move across the sky. Daylight, of course, (21) ,when the sun rises in the east and when the sun is high (22) the sky, between these two positions, half of the daylight hours have been spent.
Therefore, by noticing (23) the sun stood in the sky, early man know they could tell the time of the day. At night the motion of the stars (24) the same purpose. The important thing in keeping time is to know the exact moment of (25) . For each of us, wherever we are, noon is when the sun is directly overhead. Think of an imaginary line (26) the sky,, stretching from the north point of your horizon just to the south point.
When the sun cross your meridian (子午线), it is noon for you. When the sun still (27) of this line, or the m
A. wherever
B. where
C. how
D. when

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