Scientists are trying to find __ that they can prevent pollution. A mean B means C ways D method

这里的that个换成in which 或者省略不用.
A mean 意思
B means 方式 方法 要与介词by连用 by the means
C ways 方式途径,与介词in连用 in the ways
D method 方式 与介词with连用 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若用先行词method的花，不可以用that?只能用BY WHICH?为什么先行词只能替代名词，代词，句子等，不能替代介词你说的是什么意思，清楚点，为什么不能是find means that they can prevent pollution?