有道题答案不一致，请教！

Q4:

On Earth, among the surest indications of sunspot cycles are believed to be the rate that trees grow, as seen in the rings visible in the cross sections of their trunks.

1. On Earth, among the surest indications of sunspot cycles are believed to be the rate that trees grow
2. On Earth, among the surest indications of sunspot cycles are, it is believed, the rate of tree growth
3. On Earth, the rate at which trees grow is believed to be among the surest indications of sunspot cycles
4. Among the surest indications on Earth of sunspot cycles, believed to be the tree growth rate
5. Among the surest indications on Earth of sunspot cycles is believed to be the rate at which trees grow

   出处一：GWD31套word版第二套第4题，答案是C；出处二：gmat-prep语法笔记第59题，答案是E，针对C错误的解释是未划线部分的their指代容易产生歧义。请帮忙看看，到底应该选哪个，谢了

C is the best.  

in E, Among the surest indications on Earth of sunspot cycles   could be read as ....earth of sunspot cycles

谢谢楼上的答案，那如果选C的话，后面的their会不会产生就近指代sunspot cycles的歧义呢？

sunspot cycles 怎么会有 trunks 呢?

那 earth of sunspot cycles 怎么解释呢，我觉得也没法解释

还是没搞懂，顶一下