Letterboxes are much more ________ in the UK than in the US, where most people have a mailbox

instead. [ ]

A. common  

B. normal

C. ordinary  

D. unusual

机械密封端面宽度b过大，冷却、润滑效果差，端面磨损和磨擦功率大；而宽度b过小，端面平直度优，单位面积强度（）。

A、高

B、低

C、不变

D、理想