For each of the following, either construct a formal proof of validity or prove invalidity by means of the STTT. In each case, use the notation in parentheses.

If the linguistics investigators are correct, then if more than one dialect was present in ancient Greece, then different tribes came down at different times from the north. If different tribes came down at different times from the north, they must have come from the Danube River valley. But archaeological excavations would have revealed traces of different tribes there if different tribes had come down at different times from the north, and archaeological excavations have revealed no such traces there. Hence if more than one dialect was present in ancient Greece, then the linguistics investigators are not correct. (C, M, D, V, A)