It's great fun to go hiking in the valley, but______, we shouldn't turn a blind eye to the potential risks.   [ ]

A. on the other hand  

B. on the contrary  

C. as a matter of fact  

D. as a result

空压机每个活塞有5道活塞环，其中（）为油环。

A、1道

B、2道

C、3道

D、4道