问题
单项选择题
For a light that has an intensity of 60 candies at its source, the intensity in candles, S, of the light at a point d feet from the source is given by the formula 
, where k is a constant. If the intensity of the light is 30 candles at a distance of 2 feet from the Source, What is the intensity of the light at a distance of 20 feet from the source ？（）
A.A
B.B
C.C
D.D
E.E
答案
参考答案：A
解析：
解答此题的关键是对题意的正确理解。2英尺远处的光强为30烛光，代入光强的表达式求出k：
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则距光源20英尺的地方的光强为：
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