命题“若a>b,则2a>2b-1”的否命题为
—Why do you look so tired?
—Because we ________ for three hours without stopping.
A.work
B.worked
C.have worked
D.were working
发达国家和发展中国家的划分依据是( )
A.经济发展水平
B.面积大小
C.人口多少
D.资源丰富程度