They worked hard at___(sing).

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They worked hard at___(sing).
x) HT(/NMQH,JQH,(KԳI*'BΆS@L /.H̳js~鄾g}Kvjyܻ{zn4P*

They worked hard at___(sing).
They worked hard at___(sing).

They worked hard at___(sing).
singing

介词后接动词的分词形式,即singing