Charles Smith, ______ was my former teacher, retired last year.
正确答案： B
本题解析：
that 不能引导非限制性定语从句 , 可以用 which 和 who 引导 , 其中 , who 代替的先行词是人。句意为：查理 ?史密斯去年退休了 ,他曾经是我的老师。
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.