Oh, great. I'm off writing a response to M. Ted and Frank drops in a dissenting opinion before I can post. What's more, his looks pretty authoritative as well. So how is the difference of opinion to be resolved?
Mary didn't start out giving us any key to work from so I assumed when she said I, II, IV, V she was implying that the root of the chord (I) was also the tonic note of the key.
Frank, I don't understand. Why are you talking about the 13th? Is that another missing note from the implied chord? I figured that IV translated to the 11th but didn't see anything in Mary's question to cause me to think 13th. That's probably a result of not having any formal theory classes. And why does it have to be in C? Can't Mary's chord stand in the same key as the root of the chord?