在△ABC中，若其三条边的长度分别为9、 12、15，则以两个这样的三角形所拼成的长方形的面积是____ .

在A、B、C三个相同的烧杯内装有深度相同的液体，三种液体的密度关系是ρ₁=ρ₂＞ρ₃．将甲、乙、丙三个重力分别为G_甲、G_乙、G_丙的实心小球分别在A、B、C的液体中，其中甲球在A中沉入液体底部，乙球在B中恰好悬浮，丙球在C中漂浮在液面上．三个球的密度分别为ρ_甲、ρ_乙、ρ_丙，体积关系是V_甲＞V_乙=V_丙，三个球所受浮力分别为F₁、F₂、F₃．三个烧杯里的液体对杯底的压强分别为p₁、p₂、p₃．则以下判断的关系中正确的是（　　）

Dear Abby，

    I'm a helpless mother. My son Tom is 15 years old. I find that the older he grows, the less we talk. I feel very sad and I really need help.

    My first problem is about his hobbies. He spends almost all his spare time on computer games. When he gets home, he always turns on the computer and closes the door. I'm getting worriedabout him, especially when his English teacher tells me he always gets low marks in the English exams.

    Second, he likes newtechnology，but I don't have enough money and I can't afford all the things he wants.

    Recently, I've foundthat he is always alone. I've never seen him talking to his friends on the phone or going out with anyone on weekends. That is the last problem I have.

    What should I do to help him? Pleasegive me some advice.

A helpless mother

在果树开花的季节，如果遇到阴雨连绵的天气，常会造成果树减产，原因

探究：菜青虫总是取食十字花科植物这一行为是先天性行为吗？