The boy complained _______ a pain in his stomach.
正确答案： D
本题解析：
complain of ：抱怨，说自己有病。句意：这孩子说他肚子痛。
The professor asked a question, and David ________ a good answer.
He would have given you more help, if he ________ so busy.
After months of voyage, Columbus arrived in ______ later proved to be a new continent.
It rained hard yesterday, ______ prevented me from going to the park.
It is important to ________ your skin from the harmful strong sunlight.
He was ______ to withdraw from running for the presidency.
The two girls competed with each other ______ the highest mark.
If the operation ________ carefully prepared, it would not have been successful.