单项选择题

辐射源单位发射面积发出的辐通量，定义为辐射源的（）。

A．辐通量
B．辐射能
C．辐出度
D．辐照度

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单项选择题

38. According to the passage, if you place an ad on TV, you are unable to know________ A.
A) how often your ad is displayed
B.
B) how much your ad costs
C.
C)when your ad is shown
D.
D) who your audience i