It so happened that he________there were two uncorrected mistakes on his test paper．A.foundB.invitedC.inventedD.found out
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A解析分析：句意是“他碰巧发现他的试卷上有两个没有改出来的错误。”，是指偶然性的发现，所以选A项。(延伸)原句＝It so happened that he found two uncorrected mistakes on his test paper．＝He happened to find there were two uncorrected mistakes on his test paper．＝He happened to find two uncorrected mistakes on his test paper．(延伸)本题的“发现”，既是“偶然”的发现，也是别人尚未发现的，所以也可以用discovered(discover跟名词或从句)。