I've become good friends with several of the students in my school __________I met in the English speech contest last year.
---I’d like to buy this book, dad. ---Sorry, my darling, but the book is _____ adults only.
The two girls are so alike that strangers find ______ difficult to tell them apart.
He is busy now, otherwise he ______ go with you for a walk.
Having a trip abroad is certainly good for the old couple, but it remains _____ whether they will enjoy it.
---How long will you stay here? ---I don’t know. ________.