All of the plants now farmed on a large scale were developed from plants ____wild.

A. that once grow B. once they grow 为什么B是错的 B中的once 可以解释为一旦 为连词 从句主谓宾都齐全啊。

plants在从句中作主语 从句中的that不可省略。要是用B的话，once作为连词：解释为一旦，你可以联系一下句意：All of the plants now farmed on a large scale were developed from plants once they grow wild. 一旦变成野外的，很奇怪吧！而在A选项中出现了that以后，once就可作副词，注意哦不是连词了：解释为曾经、一度。
希望对你有所帮助吧~

选a