It is very difficult for a person to know how fast a car is traveling为什么是is travelling,原因!
问题描述：
It is very difficult for a person to know how fast a car is traveling为什么是is travelling,原因!
答
is traveling 是现在进行时
现在进行时的动词形式是：
be(am,is ,are) + 现在分词
由于主语是三人称单数,所以用 is traveling