题目

Passage 4 You may know about junk food like French fries. But do you know about junk sleep?Recently, a British survey found that electronic products in teenagers' bedrooms are affecting their sleep.The survey was done among 1 000 British kids aged 12 to 16. It found that 30 percent of them got just 4 to 7 hours sleep every day. But experts say they need 8 to 9 hours.Almost a quarter of the kids said they fell asleep more than once a week while watching TV, listening to music or using other electronic products.This is very worrying, said Dr Chris Idzikowski, a British expert. We call it 'junk s13．已知A表示点，a，b，c表示直线，M，N表示平面，给出以下命题：①a⊥M，若M⊥N，则a∥N       ②a⊥M，若b∥M，c∥a，则a⊥b，c⊥b③a⊥M，b?M，若b∥M，则b⊥a④a?β，b∩β=A，c为b在β内的射影，若a⊥c，则a⊥b．其中命题成立的是②③④．