Unit 5 课后同步练习-2022-2023学年高一英语上学期同步课文批注式解析（外研版2019必修第一册）

【语法填空】 Every autumn, millions of the North American monarch butterflies begin a long and difficult journey. Somehow they manage to travel around 4, 000 kilometres south and find their way to California or Mexico. However, until recently no one 1._____ (know) how they did this. A team of scientists 2._____ (lead) by Professor Eli Shlizerman has now found the answer. They have found out that the monarch is able to tell the time of day. It uses its eyes 3._____ (measure) the position of the sun. These two pieces of information allow the butterfly 4._____ (determine) the way to go. Finally, it manages to reach the places where it will spend the winter. 5._____ (sad), human activity is the main reason 6._____the number of monarch butterflies is falling. In many of the places where the butterfly can be found, people are destroying the 7._____ (nature) environment. They cut 8._____ trees and use chemicals that kill the plants that monarch caterpillars eat. The research on the monarch’s behav