题目

游泳时，有的人会有这样的体验：当人站立在水中且身体将要浸没时，池底对脚的支持力几乎为零。假如一位重为490牛的同学正在体验这种感觉，此时他受到的浮力约为____牛。  答案：490解析:略 — Must they clean and tidy the classroom now? — No, they      . It       after school in the afternoon.A．mustn’t; must cleanB．needn’t; must cleanC．mustn’t; need be cleanedD．don’t have to; can be cleaned