Are you carrying too much on your back at school? You're not alone. Back experts(专家) in the USA are worried about that young students are having back and neck problems because they are carrying too much in their backpacks .
    “It hurts my back when I run,” said Ebelin Reyes, a student in Virginia. “It's hard to get up the stairs with my backpack because it's too heavy.” Students have to carry heavy backpacks on their backs for a whole week’s study. Ebelin is one of them. They have   regular(固定的)backpacks with two straps(带子) to carry them, but a number of students with heavy loads(负担) have switched to rolling backpacks. The backpacks have wheels and can roll on the ground. Shirley Park's backpack weighs 10 kilos, and she said, “I'll change to a rolling backpack because I am starting to have back pain.”
     How much is too much? Experts say that students should carry no more than 10% to 15% of their own body weight. A few students have had a good idea to lighten(减轻) the load: less homework.
Doctor's suggestions:
Lighten the load. Take home only the books the students need that day.
(2) Wide straps are better. They can send out weight over your shoulders (肩)and be sure to wear backpacks with two straps.
(3) The heaviest things should be packed closest to the back.
(4) Bend(弯曲) both knees when you pick up the pack, don't just bend over the waist(腰).

已知两个不同的平面α、β和两条不重合的直线m、n，则下列四个命题中，假命题是（　　）

①扩大中等收入群体比重—缩小收入差距—释放消费红利—拉动经济增长

②扩大中等收入群体比重—纳入按劳分配的人数增多—调动劳动者积极性—经济效率提高

③扩大中等收入群体比重—高收入者减少—实现公平发展—促进社会稳定

④扩大中等收入群体比重—消费需求升级—倒逼供给侧改革—推动经济转型升级

小文在期中考试中，考了四门功课，语文78，数学83，外语81，生物比四门功课的平均分多7分，生物是____分．