Study passage context: While travelling through Tibet, the narrator sees villages with compact stone-and-mud houses, bright marigolds and prayer flags. Healthy willows and poplars look unusual because the highland landscape is generally treeless. Wood is scarce and valuable, so shrubs are harvested for fuel and sticks are stored carefully. Tibetan families also use yak dung as an ingenious substitute for wood. The passage emphasizes how homes and daily practices are adapted to a harsh, resource-scarce mountain environment. Adapted CTET source question 132: Yak dung is used by the Tibetans as a substitute for :

1. buildings
2. marigolds
3. fire
4. wood