The Hushan Great Wall site has not found city bricks except for the few residual bricks found at Site 1. Hushan Village and nearby villages have not removed the city bricks from the Great Wall for the construction of private houses. Presumably, the Hushan Great Wall uses very few bricks. What assumption does the above guess need to imply? ['No ancient brick kiln found near Hushan Great Wall site.', 'The local county records that the Great Wall of Hushan was built of rammed earth.', 'The mountains and slopes here are steep and the city bricks are difficult to transport in and out.', 'The soil here is extremely poor and unsuitable for firing bricks for the Great Wall.']