显微镜的正确使用步骤是（ ）
①取镜和安放 ②观察 ③对光 ④收镜．
A. I love you, Mum.
B. What colour is it?
C. My favourite colour is green.
D. Thank you to my teachers!
E. What's your favourite song?
一个非零有理数和它的相反数相乘之积一定是____。
