将一物体以某一初速度竖直上抛．物体在运动过程中受到一大小不变的空气阻力作用，它从抛出点到最高点的运动时间为t₁，再从最高点回到抛出点的运动时间为t₂，如果没有空气阻力作用，它从抛出点到最高点所用的时间为t₀，则（   ）

    Robby was 11 when he took his first piano lesson. I1students begin at an earlier age,2I explained to Robby. Robby said it had always been his mother's dream to hear him play the piano. So I took him as a student.

Much3Robby tried, he4the sense of tone and basic rhythm. But be dutifully reviewed his lessons.

    Over the months he tried and tried, and he'd always say, “My mom's going to hear me play someday. ”But it seemed5 He just did not have any inborn ability.

    Then one day Robby stopped coming. I was glad, as he was a bad6for my teaching!I7he had decided to pursue something else.

    Several weeks later, my students were to have a recital（演奏会）. To my surprise, Robby came, asking to play in the recital.

     “It is for8pupils, but you dropped.

     “My mom was sick. But I have been practicing. I've just got to play!” he insisted and I agreed.

    The night for the recital came. I put Robby up last in the program9I was to come up and thank all the students and play a finishing piece. I thought I could save his poor performance through my "curtain closer".

    The recital10well. Robby came up on stage. His clothes were wrinkled and his hair looked dirty. “How could his Mom?”I thought.

    Robby began and it was Mozart's work. I was not prepared for what I heard next. His fingers even11on the keys. After six and a half minutes he ended and people were all on their feet in wild12

    13and in tears I ran up and put my arms around Robby in joy. "I've never heard you play like that, Robby! How did you do it?"

    Through the microphone Robby explained :"Well , Miss Hondorf, remember I told you my mom was sick? Well.14she had cancer and15this morning. And well. . . She was born deaf, so tonight was the16time she ever heard me play. I wanted to make it17"

    There wasn't a18eye that evening. I thought to myself how much19my life had been for taking Robby as my pupil. However, he was the teacher, for it is he that showed me the meaning of20and love and believing in oneself.

下列方程中是一元二次方程的是（　　）。

当前，我国已成为网络销售第一大国，网络销售暴露出的一些问题也引起了社会各界的关注。

材料一：2011—2015年我国社会消费品零售总额、增速及消费对国民经济增长的贡献率情况表

材料二：借助大数据，网络经济能够捕捉消费信息、定制个性化商品，满足多样化消费需求，但网络假货也制约着网购消费升级。提高网络供给质量，拓展网络经济新空间，需要良好的消费环境。《消费者权益保护法》将“七日无理由退货”纳入法律中，不少商家主动配合该条款替消费者投保了退货险。国务院印发的《关于加强互联网领域侵权假冒行为治理的意见》提出，推动电子商务立法，明确网络商品交易规范、争议解决方式、法律责任，防范和打击侵权假冒违法犯罪。完善部门间执法联动机制，健全区域间执法协作机制。加强互联网领域侵权假冒行为治理，营造开放、规范、诚信、安全的网络交易环境。