that she would spend hours ( ) them.中间应该填什么

that she would spend hours ( ) them.中间应该填什么
Michel is a young girl who works for the police (as)a handwriting expert (专家).She has helped (catch) many criminals (罪犯) by using her special talents (天才).
When she was fourteen,Michel was already (so) interested in the differences in her friends’ (handwriting) that she would spend hours ( ) them.After (finished) college she went to France for a (special) two-year class in handwriting at the School of Police Science.
Michel says that it is (impossible) for people at hide their handwriting.She can discover (most) of what she needs to know simply (by) looking at the writing with her own eyes,(but) she also 是选项that she would spend hours ( ) them 这应该填什么 A writing B studying C settling D uncovering 麻烦详解

spend time on sth. 在……上花费时间
spend time with someone. 花时间与谁共度
spend time doing...花时间做

具体的要看语境和“them指代什么.