气凝胶是当今世界上密度最小的固体,一般常见的气凝胶为硅气凝胶,其密度仅为空气密度的3倍,看上去就像凝固的烟.将一块质量为3kg的硅气凝胶轻轻放在水平地面上,g=10N/kg,则它对地面的压力为( )
A:10N
B:20N
C:30N
D:40N
饭后散步已成为我们生活中最普及的一种运动,请根据以下提示,写一篇80词左右的文章.
1 When and where do you usually do it?
2 How often do you do it?
3 How long do you do it each time?
4 How does it help you keep fit?
Do you still remember the disaster(灾难)several years ago? In Indonesia (印度尼西亚) thousands of people were killed by the tsunami(海啸) that washed people out to sea and pulled children out of their parents’ arms. “The weather was fine with no clouds .There was no warning and suddenly the sea water just hit the city. In some parts the water was as high as two meters.” a man in the city said. “Sadli, an official at the hospital, said many of the dead in the city were children under the age of ten.” “Maybe the children were being carried by their hold but they fell over in the water and couldn’t hold on to their children .All the dead children drawed.” said Sadli. Officials said hundreds of houses had been swept away the force (力量) of five –meter –high waves across Aceh, which lies on the northern tip (末端)of Sumatra Island . “Tens of people saw the rushing water and didn’t run to high ground .Instead, they kept watching it .Several of my workmates are missing.” Lelana said by telephone.