This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.
Q: Passage: About 10,000 years ago, when the ice melted after the last glacial period, late Paleolithic communities were established along the lake shores and in cave systems. Evidence of human habitation has been found in caves near Vercors, close to Grenoble; in Austria the Mondsee culture shows evidence of houses built on piles to keep them dry. Standing stones have been found in Alpine areas of France and Italy. The rock drawings in Valcamonica are more than 5000 years old; more than 200,000 drawings and etchings have been identified at the site. Question: Why were houses built on piles?
A:
to keep them dry