要反映一个城市居民居住条件，应选择()指标。
A.房屋总面积
B.商品房销售面积
C.人均住房面积
D.人均住房建筑面积
参考答案：D
The book was written in 1946, ____ the education system has witnessed great changes. [ ]
A. when
B. during which
C. since then
D. since when
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