焊接管子时，对焊口表面清理有何要求？
参考答案：
焊接前，应将焊口表面及附近母材内、外壁的油、漆、垢、锈、熔渣等清理干净，直至发出金属光泽。
社会经济制度变革的根本原因是（）
A.社会基本矛盾运动
B.科学技术进步
C.国家 * * 更迭
D.人们意识形态改变
If a motorist had driven 1 hour longer on a certain day and at an average rate of 5 miles per hour faster, he would have covered 70 more miles than he actually did. How many more miles would he have covered than he actually did if he had driven 2 hours longer and at an average rate of 10 miles per hour faster on that day ？（）
A．100
B．120
C．140
D．150
E．160
