,as seen in the rings visible in the cross sections of their trunks
1. 首先看到their trunks,我就把B和D刪掉
2. 試著還原後半句
as the rate at which trees grow is seen in the rings visible in the cross sections of their trunks
C. the rate at which trees grow is believed to be among the surest indications of sunspot cycles
相同的地方省略
所以我覺得C比較好,有錯請指正
