答案：D

题目分析：考查固定词组，A. in detail详细说明；B. on average平均；C. to a degree在某种程度上；D. on the whole总的来说，句子意思“一些不希望的事发生了，但是总的来说旅行很愉快”

点评：考查固定短语的用法，需要考生牢记一些它们的用法，平时多记多积累，遇到此类题时，就很容易选择了

即学即练：After years of overwork, my father’s health ____ and he had to retire from business.

A. broke out.         B. broke up.     C. broke down.     D. broke off.

选C