A:静脉
B:动脉
C:毛细血管
D:毛细淋巴管
已知函数fx=ax2+bx+c(a,b,c∈R,a<0),满足f-1<0,f0>0,且对一切实数都有f1-x=f1+x成立,则有( )
If something is _______, it is known about by only a few people.
如图所示的四种交通工具,不用热机作为动力机械的是( )