What is an appropriate response to someone saying "merci" in French?

Read also:

• Have you ever met someone and something seemed so unusual about them but you couldn't put your finger on what it was?
• How can I handle my distrust and jealousy for my partner?
• Which bands became massively popular for covering songs rather than recording originals?
• What would have happened if Shin was a good movie instead of a bad one?
• How many girls or guys keep extra pantyhose in their glove box or console of their vehicle?
• Do guys ever want to suck a dick even though they are straight?
• Why do people turn a blind eye to bad behaviour if someone is very good looking? Whereas if someone is ugly, they get harshly judged for everything?
• Why do I want to give up on men?
• In your humble opinion, why does the narcissist mistake kindness for weakness in some people?
• Do you have any attributes quirks sensitivities abilities etc that you've come to learn most people don't experience? E.g. dream with subtext or experience de ja vu regularly or know you experience life very differently from those around you etc?
• So I’m getting piano lessons and my teacher wants me to get an upright piano instead of a keyboard. An upright piano is way above my price range, so what do I do? And what’s the difference between an upright piano and a keyboard?