My friend Tom lives on Sun Street. It is near his school. He usually goes to school on foot. But sometimes he goes to school by bike. His father is a doctor. He goes to work by car. His mother is a teacher. She teaches English. She goes to work by metro. Today is Sunday. Tom doesn't have any lessons. He wants to go to Nanjing by train.

DNA数目加倍、染色体数目加倍、细胞核数目加倍分别发生在细胞分裂的（ ）

滑动摩擦力的大小跟____ 和接触面的____ 有关．世界上第一条磁悬浮列车示范运营线已于2003年元旦在我国上海投入运行，磁悬浮列车的速度约是普通列车的三到四倍，原因是利用了磁力间相互作用的原理，是列车能在轨道上“浮”起来，从而大大减小列车与轨道间的____ ．气垫船工作原理就是船体与水面间形成____ ，使船体悬浮在水面上，从而大大的减小船体航行时的阻力．