29.of service有用的
That is, after all, the primary reason we live – to be of service to others.毕竟, 那是我们活着的主要原因——帮助他人。
30.out of service
(not being used)不在使用中；没有人在用
Some two hundred obsolete warships and submarines have been taken out of service during the past five years.在过去的五年中, 约有200艘老式军舰和潜艇陆续退役。
(not functioning)不再运行
Sorry, the lift is out of service.对不起, 电梯停用了。