为使仪器保持良好的工作状态与精度，应采用（）措施。
A、定期检定
B、经常调零
C、修理后重新检定
D、以上三种
参考答案：D
The number of people present at the meeting _________ about 1000 , a large number of whom _______ experts from abroad.
A．was; was
B．were; were
C．was ; were
D．were; was
硫化钠盐是一种易溶于液态烃而不溶于水和碱液的物质。
