We still remember once        round the famous tower when we were young.

A．having taken

B．to be taken

C．to have taken

D．having been taken

在滴定分析法测定中出现的下列情况，哪些是系统误差（）

A、试样未经充分混匀

B、滴定管的读数读错

C、所用试剂不纯

D、砝码未经校正