You are lucky to have a teacher as good as Mr.Smith.He is on

You are lucky to have a teacher as good as Mr.Smith.He is one in a million.
能说一下这句话的意思吗?
You are lucky to have a teacher as good as Mr.Smith.He is one (in) a million.
是不是他是万中无一的一个好老师?为什么不选of 或from,我选了of错了
我不太明白
还有,_______,I think,and you 'll be hit by the running train .
A.Once you take one more step
B.If you take one more step
C.One more step
D.Taking one more step
为什么选C,答案希望能够讲清楚