单选题
(All of) the plants now (raised) on farms (have been) developed from plants (once grew) wild.
【正确答案】
D
【答案解析】将once grew改为that once grew。that once grew wild是定语从句修饰plants。引导定语从句的关系代词只有在其所引导的从句中充当宾语成分时才可以省略，充当主语成分时不可省略。本句中that就是定语从句中动词grew的主语，故不可省略。