Title with question mark and subtitle

Hello. What is the correct way to separate title and subtitle when the title ends with a question mark?

a) With a question mark?
E.g.: What is life? A preliminary study

b) With a colon?
E.g.: What is life: A preliminary study

c) With both a question mark and a colon?
E.g.: What is life?: A preliminary study

I have looked on the Internet and found the three ways. I do not know if there is a correct form.

Thank you.