When I got home last night, I was frightened to find that in front of my door____.

A．sit a dog

B．sat a dog

C．was a dog seated

D．did a dog sit

分级破碎机主轴承温度过高可能原因是（）。

A、轴承损坏

B、液力耦合器泄露

C、破碎齿磨损过量

D、电控柜内接线松动