若1，2，3，a的平均数是3，又4，5，a，b的平均数是5，则0，1，2，3，4，a，b的方差是多少？

    What will our world be like in 2050? A group of experts has made____(they) predictions about our life.

    It is____(believe) that we will have “clever”cars by 2050. We won't have so many accidents b____they will probably be able to communicate with each other. Cars will slow down by themselves or stop in____(danger) situations.

    What's more we will never d____! Our brains will be put onto computers and we can live forever. The computers of____future won't look like machine. They'll look like____

    By 2050 there will be less____ (pollute). The factories will be ____(build)on the moon and other planets. We will be able to spend holidays on Mars and be back ____ spaceship in just a few days.