Tengo que responder a su carta. My answer: I have to respond to their letter. Duolingo say Wrong, it's "I have to respond to HER letter." question, why and how do they know it can only be HER. Can't "su" refer to "his letter," or "their letter," as well as "her letter." I sent off a feedback stating my position. However, is their something I missed here; any reason I could be wrong?. if I am correct i am starting to find such things very discouraging.