建筑工地上，工人用如图所示的装置将重为500N的建材从地面匀速送到6m高处，所用拉力为300N，时间为20s．不计摩擦和绳重，求：
食物所含的营养物质中，能为人体提供能量的一组营养物质是（ ）
Hello，friends! My name is Jack.I am a Chinese boy.My favourite food is chicken.I like beef noodles，too.I don't like fish but egg is OK.Potatoes are my favourite vegetable.My favourite fruit is bananas.They are so sweet.
