试述中国居民膳食指南的意义和指导大众合理饮食的基本原则。
参考答案:
14.Mum, you really need to have a. rest! You_________in the kitchen ever since you came back home three hours ago.
A.had worked
B.were working
C.have been working
D.Worked
下列关于摩擦力的说法正确的是( )
A.摩擦力大小总是与正压力成正比
B.静摩擦力是不可能使物体运动的
C.摩擦力的大小总是与接触面的粗糙程度有关
D.无论是滑动摩擦力还是静摩擦力都可以是动力