题目

Almost every machine with moving parts has wheels, yet no one knows exactly when the first wheel was invented or what it was used for. We do know, however, that they existed over 5,500 years ago in ancient Asia. The oldest known transport wheel was discovered in 2002 in Slovenia. It is over 5,100 years old. Evidence suggests that wheels for transport didn't become popular for a while, though. This could be because animals did a perfectly good job of carrying farming tools and humans around. But it could also be because of a difficult situation. While wheels need to roll on smooth surfaces有甲、乙两种物质，甲的质量是乙的12，乙的体积是甲的13，则甲的密度是乙的1616．