Every year,millions of monarch butterflies 4,000 km from Canada

Every year,millions of monarch butterflies 4,000 km from Canada
Every year,millions of Monarch Butterflies (帝王蝶) fly 4,000 km from Canada 16 Mexico and then 4,000 km 17 again.They stay in Mexico for over six months,from October to April,18 it is cold in Canada during that time.In Mexico,they 19 eggs and then die.In 20 ,the new butterflies start to fly back to 21 .There,they die 22 they lay eggs.Then the 23 butterflies fly to Mexico again.Monarch Butterflies always go to the same places in Mexico and to the 24 trees.How do they know the way?25 do they go to the same trees?Only Monarch Butterflies know.( )16.A.by B.in C.to D.over ( )17.A.there B.back C.straight D.across ( )18.A.if B.because C.but D.and ( )19.A.lie B.lay C.laid D.pay ( )20.A.April B.October C.November D.winter ( )21.A.the UK B.the USA C.Canada D.China ( )22.A.before B.later C.behind D.after ( )23.A.strong B.parent C.new D.weak ( )24.A.same B.green C.tall D.banana ( )25.A.How long B.Why C.How often D.What