----You seem to have known Dr Smith for a long time.

---- That’s true. ______ he joined our club six years ago.

A．Since

B．Until

C．When

D．Before

交会法平板仪台的最终作用是（）。

A、确定基线位置

B、观测浮标运行情况

C、勾绘水面线

D、观测计算起点距