— Why are you so popular with your students?— As a teacher, I seldom give my students so difficult a problem ____ they cannot work out.

With a lot of difficult problems ______, the newly-elected president is having a hard time.

.On April Fool’s Day people often play tricks _____ others.

.The horses________each other________the first prize in the match. 

.The Flight Company _________ its costomers top-quality service.