单选题 All of the plants now raised on farms have been developed from plants ______ in the wild.
A. once they grew B. that once grew C. they grew once D. once grew

【正确答案】 B

【答案解析】本题测试定语从句的用法。本句中所缺成分应是限定plants一词的，that代替前面的先行词plants，在从句中做主语，故不能省略，答案只能是B。