S1: The houses in the Indus Valley were built of baked bricks. P: This staircase sometimes continued upwards on to the roof. Q: Access to the upstairs rooms was by a narrow stone staircase at the back of the house. R: The drains were built in the walls. S: The houses had bathrooms and water closets, rubbish chutes and excellent drainage systems. S6: They led outside into covered sewers which ran down the side of the streets. Find the correct sequence of P, Q, R and S.