求证：四边形ADCE是菱形．

要合成带有放射性氧元素（*O）的乙酸乙酯，除必要的反应条件外，下列各组反应物能满足合成要求的是（　　）

①CH₃CO*OH和C₂H₅OH             

②CH₃COOH和C₂H₅*OH

③CH₃C*OOH和C₂H₅OH　　　　　　　

④CH₃CO*OH和C₂H₅*OH．

阅读下面短文。从短文后所给的A、B、C三个选项中选出最佳选项。

Dear friends,

    My name is Sarah and I am 23 years old. I am a 1 in a hospital and I would not change my career choice for anything else in the world 2 I love helping people who are injured and helping them live a healthy life. Just the simple "3" I receive from a patient or a family member makes 4 worth it.

    I love the outdoors, anything like horseback riding or boating! I may not be a common girl because I love 5. I love watching and doing sports!

    I believe that laugh can make any day better! That is why a sense of humour (幽默感) is a MUST for me! I love to make people 6 and I can be very funny when needed! That is also why I really enjoy 7 to comedy clubs like the Funny Bone. What's more, I love country music and I often go to some concerts.

    My friends and family 8 everything to me and I would be sad 9 them. I love them very much. I love 10 time with my brothers and sisters. That's me. Do you want to make friends with me?   

Yours,

Sarah

步骤1：将两个磁铁按图甲放置，手拿住上面的磁铁，将下面磁铁的两端分别接触上面磁铁的下端，下面磁铁均掉落。

步骤2：将两个磁铁按图乙放置，手拿住上面的磁铁，下面的磁铁不会掉落。