English and the Austronesian language Mbarbaram both use the word "dog" for canines. These two languages are unrelated, and since speakers of the two languages only came in contact with one another long after the word "dog" was first used in this way in either language, neither language could have borrowed the word from the other. Thus this case shows that sometimes when languages share words that are similar in sound and meaning the similarity is due neither to language relatedness nor to borrowing. The argument requires that which one of the following be assumed? ['Usually when two languages share a word, those languages are related to each other.', 'English and Mbarbaram share no words other than "dog. "', 'There is no third language from which both English and Mbarbaram borrowed the word "dog. "', 'If two unrelated languages share a word, speakers of those two languages must have come in contact with one another at some time.']