she began practicing her guitar skills for several hours each day,__when her fingers began to

虽说C,D 都可以作结果状语, 但only to do...一般表示“意料之外的结果”. only doing sth 一般表示“意料之中的结果”
分析句子： 每天练吉他好几个小时, 在手指出血的时候歇一歇应该是“意料之中的结果”, 故选D

比较：
He hurried to the station, only to find the train had left.

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