After class, I often ______ music. It's very relaxing.    [ ]

A. listening    

B. listen    

C. listen to      

D. listens to

某受压细长杆件，两端校支，其临界力为为50kN，若将杆件支座形式改为两端固定，其临界力应为（）kN。

A.50 

B.100 

C.150 

D.200