根据短文理解, 从A、B、C、D四个选项中选择最佳答案填空。

    There are robots all around us. Some do very 1   jobs like flying planes and driving subway trains. And some do simple jobs.

    When an automatic自动的) washing machine   2 , water goes in. The machine wait until the water is hot  3   washing the clothes. It does this by "feedback(反馈)". Information about what's happening is fed back into the robot to tell it   4  to do next.

    Our eyes, ears and other senses are our feedback. They tell us   5  around us. So robots are like people in   6 ways. They work and they have feedback.

    But   7   robots look like people. Many are hidden away. Robots control the temperature o f our houses, our cookers, our hot water systems.  We can set the controls to the temperature we want. The robot does 8  . Its feedback usually comes from a thermostat（自动调湿器）.

    One kind of thermostat is a strip(细长片) of metal, which bends(弯曲) when it gets hot. At the right temperature, it bends just enough to work a switch. This turns   9 the heat. As the air around it gets 10   , the metal gets straight, and this turns the heat on again.

   11－9=（  ）