CharlesSmith,______wasmyformerteacher,retiredlastyear.
B
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。
If the operation ________ carefully prepared, it would not have been successful.
Thenewdiscovery_______beofgreatservicetomankind.
He would have given you more help, if he ________ so busy.