求解..., Ving 的用法？

zhtchasedream

OG12,1 C答案为什么是错误的？
原句：The Glass House Mountains in Queensland, Australia, were sighted in 1770 by the English navigator Captain James Cook, by whom they were named supposedly because its sheer wet rocks glistened like glass.
c：naming them supposedly since their

看OG的解释是：Captain James Cook不是句子的主语，他是by的介词宾语。所以-ing不是修饰James Cook.

不大明白为什么，求大牛帮忙，同时，简单讲解下这种Ving 的用法？