PREP 64

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

 答案A，中的ｔｈｅｉｒ没有修饰问题吗？并且应该是the usage导致严重的环境问题啊。如果是increase应该的导致的环境问题的增长啊

又抓狂了

their 指的是 synthetic fertilizers

the use of synthetic fertilizers = their use

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.