植树。龙龙栽了7棵,明明栽了12棵,还要栽30棵。
(1)明明和龙龙一共栽了多少棵树?
(2)这次一共要栽多少棵树?
(1)12+7=19(棵)
(2)19+30=49(棵)
You look very tired. Why ______ and have a rest? [ ]
A. not stop
B. not to stop
C. don't stop
D. not stopping
常温下,已知:4Al(s)+3O2(g)="===" 2Al2O3(s)ΔH1、
4Fe(s)+3O2(g)="===" 2Fe2O3(s)ΔH2,关于ΔH1、ΔH2的比较正确的是( )
A.ΔH1>ΔH2
B.ΔH1<ΔH2
C.ΔH1=ΔH2
D.无法计算