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重点讲解：
1. take part in 参加；参与；
He took part in the organization of a trade union.
他参加了工会的组织工作。
2. sign up for (和…)签约；雇用；报名(参加课程)；
She's decided to sign up for music school.
她已决定报名上音乐学校。
3. due to 由于；因为；
The train had stopped due to a mechanical problem.
火车因为机械故障停了下来。
4. roll out 推出；
Once approved, the company will roll out a next-day delivery service and second and third-day delivery products.
一旦获批，该公司将推出次日送达服务以及第二日和第三日送达产品。
n. 媒体，方法，媒介
adj. 适中的，中等
