根据题意,1-3x<0,
移项得,-3x<-1,
系数化1得,x>13;
所以当x>13时,分式-21-3x的值为正数.
能杀伤细胞的细胞因子是()
A.IL-2
B.TNF-α
C.干扰素
D.IL-4
E.IL-1
David can speak idiomatic American English and yet, _____ English, he doesn't know it at all.
A.how to teach
B.when it comes to teaching
C.when he teaches
D.however he teaches