1. A method to select advertisements in response to an internet query, the method comprising: receiving an internet query (Q), the internet query including query terms (q); retrieving and ranking a first set of ad materials (S) in response to the internet query using a query likelihood model, S including a plurality of ad materials (s); selecting sampling words using pseudo-relevance feedback and translation models, the internet query, and the first set of ad materials, wherein the sampling words are chosen from a distribution of words from the words in S, wherein the pseudo-relevance feedback model is used to select a word (w) in the distribution of words based on a probability that w generates q(p(q|w)), wherein the probability p(q|w) is based on a translation probability that w translates into q(t(q|w)), t(q|w) being calculated with the translation model; retrieving and ranking a second set of ad materials using an expanded query, the expanded query including the internet query and the selected sampling words; and displaying advertisements based on the second set of ad materials.