二次函数y=x2+4的最小值是______．
二次函数y=x2+4最小值为y=4ac-b24a=164=4．
I know him ______ a good football player while in college.
A．to have been
B．to be
C．was
D．had been
说出硫有几种同素异形体存在，并指明存在这种同素异形体的温度。
