3. After a slow sales start early in the year, mobile homes have been gaining favor as ___ to increasingly expensive conventional housing.
(A) reaction
(B) an addition
(C) an introduction
(D) an alternative
(E) a challenge
选D
翻译：尽管年初经历了一阵滞销，但随着传统住宅价格持续上升，移动房屋(一般指能用车拖的住房，也可固定在地上)作为另一种选择越来越受到青睐。
有可能是这样的(我也没见过)：
思考题：为什么不能选 challenge 呢?
