We grease the bearings of all pumps at intervals.我们定期给所有泵的轴承加牛油 01010038
A.
We grease the bearings of some pumps periodically.
B.
We grease the bearings of several pumps periodically.
C.
We grease the bearings of only one pump periodically.
D.
The bearings of all pumps are greased periodically.