Just tried the step suggested by these guys.
Well, the security questions are really dumb! I entered an e-mail address of a friend and entered the birthday. Then I was ask: what is your hometown? WTF everybody knows this guy's hometown! I didn't go any further but it made me very worried about my security questions. I'd better enter some password like stuff in the answer fields.
Then set up 2-factor authentication. There will no longer exist any security questions for your account. Problem solved.