S1: The houses in the Indus Valley were built of baked bricks.

S6: They led outside into covered sewers which ran down the side of the streets.

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.

The proper sequence should be

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