—Don't you usually go to work by bus?

— ___________.  But sometimes l walk.  [ ]

A.  Yes, I do    

B.  No, I don't

C.  I take a bus    

D.  I take the subway

活塞杆承受（）载荷，因此活塞紧固在活塞杆上必须保证（）可靠。