答案是A。 但不明白为什么C不对，请问：C答案具体有哪些错误呢？

64. Whereas the use of synthetic fertilizers has greatly expanded agricultural productivity in many parts of the world, an increase in their use can create serious environmental problems such as water pollution, and their substitution for more traditional fertilizers may accelerate soil structure deterioration and soil erosion.
A. an increase in their use can create serious environmental problems such as water pollution, and their substitution for more traditional fertilizers
B. an increase in their use can create serious environmental problems like water pollution, and if substituted for more traditional fertilizers, it
C. if these fertilizers are used increasingly, they can create serious environmental problems such as water pollution, and if used as substitutions for more traditional fertilizers, they
D. the increased usage of these fertilizers can create serious environmental problems such as water pollution, while if substituted for more traditional fertilizers, this substitution
E. the increased usage of these fertilizers can create serious environmental problems like water pollution, while their substitution for more traditional fertilizers

whereas连接的部分最好形式对称，A比C优选。而且我觉得这里用名词短语an increase in their use, their subsitution for..来代替if从句更简洁明了。C没有绝对的语法错误，只是从优选的角度排除。至于prep上解释说的substitution 而不是fertilizer本身造成后果，我倒认为不太能成为排除的原因。

我觉得是"简洁"问题更突出得表现在这道题里.