response to higher oil prices, window manufacturers have improved the insulating capability of their products; their windows have been built to conserve energy, and they are.
-
Ahave been built to conserve energy, and they are
分析该选项
-
Bare built to conserve energy, and they have
分析该选项
-
Care built to conserve energy, and they do
分析该选项
-
Dare being built to conserve energy, and they have
分析该选项
-
Ehad been built to conserve energy, and they are
分析该选项
显示答案
正确答案: C