So far very impressed with Duolingo but seems very much "nit picking" with the various types of "Thank you" when neither is a mistake why mark it as an error!
Mersi is informal, mulțumesc is both formal and informal (used more in formal contexts). Since "thank you" is neither formal nor informal, both should be accepted.
Funny. The sentence before was corrected by "multumesc" when I wrote "merzi" and now this one needs merzi istead of multumesc?? Are ypu all confused or trying to be confusing?!
N importe quoi!!! ( sorry I Am FRENCH)