语法与词汇 By Tuesday, Charlie’s back had improved, and he pitched in with Seebees in the worst volunteer work of all.无

【正确答案】“Pitch in” means to join in and to help with an activity.

【答案解析】句意：到周二，查理的背恢复了，他和西贝一起加入到最艰苦的志愿服务中。 pitch in 在这里就指“投入某一活动并在其中做贡献”。