问题描述：
drop在这里为什么要用过去分词?
Happiness is like a pebble dropped into a pool to set in motion an ever-widening circle of ripples.As Stevenson has said,being happy is a duty
这里的drop是因为是被动的掉下去才用dropped的吗?
还有第2句的Stevenson has said为什么要用现在完成时,而不是用过去完成时?
Happiness is like a pebble dropped into a pool to set in motion an ever-widening circle of ripples.As Stevenson has said,being happy is a duty
这里的drop是因为是被动的掉下去才用dropped的吗?
还有第2句的Stevenson has said为什么要用现在完成时,而不是用过去完成时?
问题解答：
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