Perhaps the days will come ____ people will be able to breathe clean air in cities.
A. as B. when C. while D. sine
出自:统招专升本-----英语

完成拼图验证

按住下方滑块,将拼图拖到缺口处后松开。

拼图背景,将小拼图拖到缺口处
向右拖动滑块