8086和80286都有4个段寄存器CS、DS、SZ、ES，它们都是_______位的寄存器，分别是代码段寄存器，数据段寄存器，堆栈段寄存器、附加段寄存器。它们用于内存寻址时， 20位的物理地址是由段寄存器左移四位和16位的偏移量相加而成。

There is a big library in our school.There are more than twice________five years ago. [ ]

A. books than  

B. as many books as

C. many books as  

D. many more books than