将下列句子改写成定语从句或者含 v-ing/ v-ed 的句子。 1. To find an answer, scientists have built mathematical models of food webs, and note who eats whom and how much each one eats. 2. With such models, scientists have found out some key principles that operate in food webs. 3. Scientists have built an early-warning system which is based on mathematical models.