I have some doubts about the answer.

I'm currently doing some questions from a FCE practice test and I don't know an answer  (or the answer to one of the questions). I'm talking about the use of English and the sentence says: It was impossible for me to know which road to follow. In this case, i need to write one more sentence using "Not" and the sentence says It .... known which road to follow . I cannot use more than 5 words and i need to include not. Does someone know the answer?

It is not known which road to follow

Thanks in advance.