下列实验操作不当的是

A．将烧碱放在已知质量的烧杯里用托盘天平称量

B．将试管夹从试管底部往上套，夹在试管的中上部

C．将鼻孔直接凑到盛有二氧化硫的集气瓶口闻气味

D．量取液体体积读数时，视线与凹液面的最低处保持水平

—Didn't the teacher see him entering the classroom?    

—No, he_______ on the blackboard.

A．was writing

B．had written

C．wrote

D．is writing