The classroom is big enough        , but we’ll have to move if we have more students. 

--What about the protection? ---Let’s keep the surface ____ dirty by putting a cover over it.

Although ______ for many years, the two old sisters recognized each other at once.

——You don’t go to that supermarket quite often, do you? —— No, I only go there ______ because it’s too far away from my house.

Only when __________________ permission _______________ allowed to enter the lab.