北魏时出现的我国古代杰出的地理学家是[ ]

A、祖冲之    

B、郦道元  

C、贾思勰    

D、郭守敬

It seemed that our nursery rhymes___ some magic because all the nights I sang them for my daughter, she would listen to them with keen pleasure．

A．must have had

B．should have had

C．could have had

D．need have had