John was so sleepy that he could hardly keep his eyes .A. openB. to be openedC. to openD.

问题补充：

John was so sleepy that he could hardly keep his eyes .

A. open

B. to be opened

C. to open

D. opening

答案：

(答案→)A. open

B. to be opened

C. to open

D. opening

解析：作keep的宾语补足语，表状态用形容词或过去分词；当既有过去分词，又有相应的形容词时，补语用形容词。