First off there's a good chance they won't even be brought up or if they are in very brief passing.
But if it is brought up, it's really not that hard to answer.
1) Acknowledge responsibility for prior poor grades
2) Talk about what you did to turn your performance around and what you gained in the process.
There are far bigger flaws an applicant, even a competitive one can have, then a few underclassmen C's with a 3.7 overall. Just own those grades rather than thinking those C's will do you in and there shouldn't be any issues.