若要选C.此时as用作连词,连接一个时间状语从句,意为“当~时候”：即
As he was reading the book,he found out some answers to this question.故C不对.
on/upon为介词,此处意为“一~就”,后接名词或动词的ing形式.
On/upon reading reading the book,he found out some answers to this question.
由于后跟的reading,所以只能选D.
____ the book,he found out some answers to this question.
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