-----What is your brother like ？

-----_______________[ ]

A.He is fine, thank you.

B.He is doing his homework now.

C.He is tall and thin.

D.He is a famous doctor.

由于铁磁物质的起始磁化曲线不一条（），因此铁磁物质的磁导率μ不是（）。μ值是（）的函数