如图，△ABC中BC边上的高为h₁，AB边上的高为h₂，△DEF中DE边上的高为h₃，下列结论正确的是（   ）

交通运输布局的变化会对商业网点分布产生显著的影响。近年，以沃尔玛、好又多(诚达)、国美、苏宁等为代表的大型超市进驻厦门岛内，形成新的商业网点格局。读图，回答下列问题。

看拼音，写词语。

cóng  ér	zài  jiàn	chuāng wài
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假设你叫林洋，收到了你的笔友Susan的一封信，请你给她写一封回信。在信中回答她的问题，提出你的建议，并就此事谈谈你的看法。
Dear Lin Yang,
I have a problem these days. My mother doesn’t allow me to watch TV even on weekends. She says as a middle school student, my job is to study. After I finish my homework, she says, I should read books and watching TV is bad for me. But I think I need a rest and watching TV helps me learn a lot too.
Do you have the same problem? What do you think of my mother’s decision? What should I do?
Yours,
Susan

某放射性元素质量为M，测得每分钟放出1.2×10⁴个β粒子，21天后再测，发现每分钟放出1.5×10³个β粒子，该放射性元素的半衰期是多少？