下列物质在常温下氧气中燃烧，都可以生成无色、无味气体的一组是[ ]
A．硫粉、红磷
B．铁丝、硫粉
C．木炭、一氧化碳
D．氢气、木炭
答案：C
The construction of the two new railway lines ________ by now.
A．have been completed
B．has been completed
C．has completed
D．have completed
简述民族的迁移与集聚。
