The solution to the mystery of the monarch’s amazing ability comes at a time (4)when it is in serious trouble.Its population has crashed by as much as 90 per cent in the last few years.Sadly, human activity is the main reason (5)why the number of monarch butterflies is falling.In many of the places where the butterfly can be found, people are destroying the natural environment.They cut down trees and use chemicals that kill the plants (6)that monarch caterpillars eat.