The older a tree is, the more its rings are. The locust trees in Lao Zhang's yard are older than those in Lao Li's house. Therefore, the locust trees in Lao Zhang's house have more rings than those in Lao Li's house.
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The higher the altitude, the thinner the air. Because the altitude of Xining is higher than Xi'an, the air in Xining is thinner than Xi'an