I can’t______ the difference betweenthem. Which one is wrong? [ ]

Would you please ____ _ her _____ me back— All right, I will tell her.

The wreckage of something like a jet plaindiscovered in the heart of the desert is believed ______ a UFO and _______downfor reasons still mysterious.

They _______ arrive at the station before 12 o’clock, but they don’t appear now.

How I want to have learnt English well when I was at the college. —If you had, you ______ in the U S A at present. [ ]