我国的化学试剂等级标准分为()级。
A.二
B.三
C.四
D.五
参考答案:C
—Bad luck! It's too late!
—It doesn't matter. The movie _____ for only a few minutes.[ ]
A. began
B. has begun
C. has been on
光纤接续要求纤芯熔接损耗平均绝对值小于()dB,确保熔接质量,不合格的需返工。