答案：C

题目分析：考查连词和省略句：A. as当…时候，因为，正如，B. unless除非，C. if如果，D. before在…前面，句意：上个月这个城市的房屋销售下降了40%，如果不算上为贫穷居民提供的廉价房是48%。这句话是省略句，完整的是：The sales of houses in our city fell by 40 percent last month, The sales of houses in our city fell by  48 percent if we don’t count low-price houses provided for poor residents.选C。