fast比较级和最高级怎么写

　　fast的例句：She climbed the staircase cautiously，holding fast to the rail（她紧紧地抓着栏杆，小心翼翼地爬楼梯）；Brindley was known as a very，very fast driver（众所周知，布林德利是个喜欢飞车的人）。