A:what my teacher say
B:what does my teacher say
C:what my teacher said
D:what did my teacher say
关于巴西的世界之最说法错误的是( )
下列词语中划线字的读音完全正确的一项是( )
用规格相同的滑轮安装成甲、乙两个滑轮组提升相同重物,如图所示,滑轮与轴的摩擦及绳重忽略不计.甲的机械效率为η1,乙的机械效率为η2,则下列关系正确的是( )