— ______ people travel every year?
— Perhaps when we are talking about it, more than 100 planes have taken off around the world.
A：How many
B：How much
C：How long
D：How often
①会抑制人们的消费需求，不利于经济发展
②有利于节约资源，保护环境
③反映出人们践行绿色消费原则
④有助于减少消费支出，降低恩格尔系数
Many a student __________ wearing MP3 players while walking on the street． It's quite dangerous． （ ）
