已知:a>b,则下列不等式一定成立的是( )
A:a+4<b+4
B:2a<2b
C:-2a<-2b
D:a-b<0
在海边观察远处驶来的轮船,会出现下图中①至③所示的现象,这种现象反映的地理事实是( )
句子改错
He came into the classroom and the students stopped to talk.
____
下列行为中,属于有教养的表现是( )
-- You seem familiar with Mr. John.-- Of course, we_______ together for three years in the 1990s.
若变量x,y满足约束条件x+y-3≤0x-y+1≥0y≥1,则z=y-2x的最大值为( )