By the time Jack returned home from England, his son  _______  from college.

A．graduated

B．has graduated

C．had been graduating

D．had graduated

同一组（），宜设置在同一断面线上。当因地形限制或其他原因不能设置在同一断面线时，其最上游与最下游水尺的水位落差不应超过1cm。

A、比降上断面水尺

B、比降下断面水尺

C、基本水尺

D、比降水尺