矿井的反风量不得少于正常风量的（）。
A.30%
B.40%
C.50%
参考答案：B
With the development of high technology, more and more new things ______ in the past years.
A．discovered
B．have discovered
C．had been discovered
D．have been discovered
病毒粒有哪几种对称形式？每种对称又有几种特殊外型？
