5. 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.A. extendedB. amountedC. addedD. turned

步骤 1：理解句子结构
句子的主干是“he still found it hard to understand this point”，其中“Although he had looked through all the reference material on the subject”是让步状语从句，表示尽管他查阅了所有相关资料，但仍然难以理解这一点。后半句“her explanation only ________ to his confusion”表示她的解释只是增加了他的困惑。
步骤 2：分析选项
A. extended：延伸，扩展，不符合语境。
B. amounted：达到，总计，不符合语境。
C. added：增加，添加，符合语境。
D. turned：转变，不符合语境。
步骤 3：选择正确答案
根据句子的语境，她的解释只是增加了他的困惑，因此选择“added”。