设定点F₁(0，－3)，F₂(0,3)，动点P(x，y)满足条件|PF₁|＋|PF₂|＝a(a>0)，则动点P的轨迹是(　　)

    I have two good friends．They are Gina and Jenny, and they are twin(双胞胎之一的)sisters．They come from Singapore．They are 12 years old．They can speak English and Chinese．

    Now the twins are in China．Their family are in China，too．They live in Shanghai．There are five people in their family．They are their parents，their brother and them．Their mother is a doctor．Their father is a teacher．Their brother Alan is very young．He is four years old．He can't go to school．

    Gina，Jenny and I are in the same class．Every day, we walk to school and talk a lot．They say China is a very good place and they like China．

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