抛物线y=ax²的准线方程是y=2，则a的值是（　　）

    While waiting for my daughter I treated myself with a coffee which I used to be so dependent on that when I stopped it I had headache for days. So while I was enjoying my rate moment, I was attracted by an old men who seemed not well

    I could not see his face to be sure, as it was laid between his folded arms on the table. He had a long warm coat, and a big backpack which made me think he could be a homeless. I quit wondering, went to his table, patted him on the arm and asked him, “Are you OK?” He raised his head and said something in a whisper, nodding yes. I realized by the strong smell that he was indeed a homeless man.

    I asked him if he was hungry. He responded but I could not understand his answer. However, I didn't give up and asked him if he would like some rice balls at which he raised his head saying clearly this time that he would be thankful for a rice bait Immediately I went to the nearest shop and bought a drink and a warm lunch box, and brought them to him. He said “thank you” but didn't eat right away and made no move, maybe because he was a little shy in the presence of a lady.

    So I left them on his table and changed my waiting place. When I passed again nearby, he was eating. You should have sensed my great pleasure. I was so grateful that I had the ways to help an old man even if only for a moment of his life.

①若甲容器中水的体积为4×10^﹣3米³，求水的质量m_水；

②若h等于0.1米，求水对容器底部的压强P_水；

③现沿竖直方向在圆柱体乙上切去底面积为S的部分，并将切去部分竖直置于容器甲的水中后，自然静止沉在容器底部，此时甲容器对水平桌面的压强P_容′与切去后的乙对水平桌面的压强P_乙′之比为5：8，求圆柱体乙的密度ρ_乙。