這里的that個換成in which 或者省略不用.
A mean 意思
B means 方式 方法 要與介詞by連用 by the means
C ways 方式途徑,與介詞in連用 in the ways
D method 方式 與介詞with連用 with the method.
Scientists are trying to find _ways_ that / in which/ / they can prevent pollution
Scientists are trying to find _means _by which they can prevent pollution
Scientists are trying to find _method_ with which they can prevent pollution若用先行詞method的花，不可以用that?只能用BY WHICH?為什么先行詞只能替代名詞，代詞，句子等，不能替代介詞你說的是什么意思，清楚點，為什么不能是find means that they can prevent pollution?