An inferior good is one for which demand decreases as income increases. This is different from a luxury good, which is a good for which demand increases more than proportionally as income increases. The fact that x1 is an inferior good does not imply anything about the nature of x2; it could be a normal good, a luxury good, or even another inferior good. However, the question implies a dichotomy between x1 and x2, suggesting that if x1 is inferior and we are to make a conclusion about x2, then by elimination of other options, x2 could be a luxury good.