设集合A={a,b},B={-1,0,1},则集合A到集合B的映射共有几个?请将它们分别写出来,可是

What can be inferred about the man from the conversation？

A．He always registers early.

B．He usually registers at the last minute.

C．He knew the woman would be in the registrar"s office.

D．He was late for paying his tuition.

请帮忙给出正确答案和分析，谢谢！

Why is the woman at the registrars office？

A．She works part-time there.

B．She goes to apply for a job.

C．She goes to sign up for classes.

D．She goes to pay her tuition.

请帮忙给出正确答案和分析，谢谢！

简述羊的同期发情技术。

宁夏的滩羊传说是“吃的中草药，喝的矿泉水”，其属短脂尾羊，是我国珍贵的裘皮羊品种，所产二毛裘皮独具一格。()

母羊通常在什么时候排卵？