滚动轴承的内外圈和滚动体用哪种材料制造（）
A.耐高温，防腐蚀的铝锰高碳钢制造
B.强度高，耐磨性好的铝锰高碳钢制造
C.抗疲劳性好的合金钢
参考答案：B
–What’s the noise?
–It’s the drill. Since the beginning of the term, the workers _______ our heating system.
A．are repairing
B．have been repairing
C．will be repairing
D．repair
