(2005)对于平面应力状态,以下说法正确的是:()
A.主应力就是最大正应力
B.主平面上无剪应力
C.最大剪力作用的平面上正应力必为零
D.主应力必不为零
参考答案:B
With the help of high technology, more and more new substances ______ in the past 10 years.
A.discovered
B.have discovered
C.had been discovered
D.have been discovered
热水循环流量