Although he had looked through all the reference material on the subject, he still found it hard to understand this point and her explanation only______to his confusion.
【正确答案】 C
【答案解析】解析：题干：尽管他已看过所有与该主题有关的参考资料，他仍然发现难以理解这一点，并且她的解释只会增添他的困惑。此处四个单词均可与介词to搭配，但搭配后的意义完全不同：extend to延伸至。amount to合计，总共达，相当于，等于。add to增添，做加法。turn to转向某人寻求帮助。因此，只有add to符合题意。