Don't open the door. It's cold. Please ____ the door.

地方性甲状腺肿是一种常见偏远山区的营养缺乏症，对患者及其后代有严重的危害。目前较有效的防控措施是在食用盐中添加(  )

下列说法正确的是（　　）

阅读理解

    On a small farm in Mexico, there are no schools. A bus is the school. The driver of the bus is the teacher. It's a school bus, but it doesn't take the children to school. It just goes round from place to place, and sometimes it comes to this farm. The bus will stay here for three months. The farmers call it a school on wheels (车轮). Every time when the bus comes, the farmers will come, running to it, shouting and laughing. They warmly welcome the school bus.

    When the bus is on the farm, in the morning, the teacher teaches t he small children. In the afternoon, the bigger children come to have their lessons because they must work in the morning. At night, the fathers and mothers come to school. They want to learn, too. How the farmers hope that some day they can have a real school on their farm!