— It's a very interesting book.    

— _____. And thank you very much for letting me keep it so long.       [ ]

A. I'm glad you like it        

B. That's all right      

C. Don't mention it            

D. I hope you like it

什么是张拉控制应力σ_con，为什么张拉控制应力取值不能过高也不能过低