正常服务以外，双方通过书面形式协议附加于正常服务的工作内容，属于（）。

A.附加服务

B.额外服务

C.辅助服务

D.补充服务

Born ten days earlier, the boy ______ his late father.

A．could have seen

B．must have seen

C．may have seen

D．should have been