鸦片战争前，中国社会经济中占统治地位的是（）。
A.商品经济
B.自然经济
C.半殖民地经济
D.资本主义经济
参考答案：B
Rarely _____ by car at that time. Their parents were too poor to buy one. [ ]
A. they went to school
B. did they go to school
C. they didn't go to school
D. did they not go to school
排水系统综合效率，斜井不低于（）。
A.30%
B.40%
C.50%
D.75%
