Dear Dad,

I know you were very busy with work, but I'm hoping if you can help me out anyway. I’m thinking of leave the football team, and I need your opinion. My homework this year is crazy, but I need more time to get them done.Since I’m the man of the house while you’re gone, I have much housework, either. On the other hands, I’ve put in two years on the team, and the coach says I’ll make captain soon. However, I’d like try something new. I’ve been thinking about photography, actual, and I want to meet some new kids. How do you think, Dad? I can’t wait to see you!

Yours,

Xiao Liang

由于正火较退火冷却速度快，过冷独度大，转变温度较低，获得组织较细，因此同一种钢，正火要比（）的强度和硬度高。

A、正火

B、退火

C、回火