Why does dry HCl gas not change the colour of the dry litmus paper?

Q. Why does dry HCl gas not change the colour of the dry litmus paper?

1. Because dry litmus paper is not an indicator
2. Because HCl gas is a weak acid, we must use a strong acid
3. Because dry HCl gas does not ionize to produce H+ ions
4. None of the above

Answer: Because dry HCl gas does not ionize to produce H+ ions