PP1-124 their 的指代问题

124. (31091-!-item-!-188;#058&004372)
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

关于A选项中的their, 根据句意可以知道是指代fertilizers，但我觉得GMAT中代词只能指代中心名词，那这里变成只能指代parts了，不懂？有谁能帮忙解答一下吗？？
谢谢！！