—According to my mother, it is a good idea to have some chicken soup when you have a cold．

—_____, scientists agree with her．

A．Sooner or later

B．Once in a while

C．Believe it or not

D．To be exact

钢结构用钢材机械性能指标有哪几些？承重结构的钢材至少应保证哪几项指标满足要求？