题干

有下列四个命题:①若z∈C,则z2≥0;②若a>b,则a+i>b+i;③若xy∈R,则xyi=1+i的充要条件为xy=1;④若实数a与复数ai对应,则实数集与纯虚数集一一对应.其中正确命题的序号是____.
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阅读下面短文,从短文后各题所给的A、B、C和D四个选项中,选出可以填入空白处的最佳选项。

    A student at Hellesdon High School in England has been called “Rubbish Girl” by bullies (欺凌者) for her environmentally-friendly ways.1, that has not stopped her from cleaning up the environment.

    Using the basket on her bicycle, 12-year-old Nadia has been 2 rubbish along her two-mile route to and 3 school since September of last year.4her good intentions, some of the kids at Nadia's school have nicknamed(给……起绰号) her “Rubbish Girl” and have bullied her for her selfless efforts to help the 5. But instead of letting them bring her down, she's using their 6as motivation(动力).

    “I'm not going to 7 doing the right thing because of them,” she said. “I'm doing something to 8 the world which they also live in. It's everyone's job. We are all 9for keeping this world safe, instead of 10 it's always someone else's job.”

    Nadia's mom, Paula, said she is11 proud of Nadia for not only standing up to her bullies, but also standing up for what she believes in. “I told her she had two 12 — she could either stop collecting rubbish, stop attracting their 13 so that they would leave her alone, or she could face the challenge, be the “rubbish girl” and keep her 14

    As a result of the local 15 attention, Nadia has created a Facebook group named “Team Rubbish Girl”,16she shares updates on her green efforts. Positive 17 have poured in, all in 18 of Nadia, encouraging her to carry on with the green effort.

    “Sweetheart, you are a star that 19 really bright above the rest. Hold your head high and never let others 20 you. You are doing something selfless and amazing. Shame on the bullies.” wrote one of the local citizens on her Facebook page.