Read the passage given below and answer the question that follow: Every now and then we would speed through a village lined with waving Tibetan children. Their villages looked wonderful and so inviting that Dorje was not showing any signs of slowing down. Small clusters of single and double storey buildings with walls-in courtyards jostled together in the foothills to gain maximum exposure to the sun. The houses looked solid, built for the harsh environment. Walls are made up of stone to waist height and finished off with mud bricks to the roof. Tin cans lined the window ledges, with the bright orange of marigolds in full bloom livening up the black-and-white of the houses. Branches of trees adorned with colourful prayer flags stood high into the wind from the top of the flat roofs. The auspicious blue, white, red, green and yellow colours of the fabrics stood out against the rich blue of the Tibetan sky. Each prayer flag carries a picture of Lungta, the jewelled dragon horse, who carries the owner's prayers up to the divine every time the flag flaps in the wind. The larger villages have healthy trees, usually willows or poplars which look quite out of place in the generally treeless landscape. Wood is a precious commodity in the highland areas of Tibet and is never wasted. The few shrubs which grow wild on the hillsides are harvested for use as brushwood and each courtyard wall is piled high with sticks gathered from the mountains to get a fire started. The lack of solid fuel in the shape of wood is not a problem for the Tibetans, who have an ingenious wood substitute: yak dung. What aspect of Tibetan houses as described in the passage is incorrect ?