结构推理 他一直在琢磨这个问题，直到把它完全解决为止。(to chip away at)
【正确答案】He had been kept chipping away at the problem until he had solved it completely.
【答案解析】[翻译指导] “一直”可用keep doing来表示。表示一直做某事直到过去的某个时间为止，最好用过去完成时。