When asked, the naughty boy hang his head and unwilling to tell his mother ________ he had done in the park yesterday afternoon．

A.that

B.how

C.where

D.what

答案及解析： D

这是一个宾语从句。宾语从句中he had done后缺少宾语，所以要填what。另外，如果后面有宾语而成为完整的结构，则B项也可以考虑。