He ran all the way to the station only _______________the train had already left

We can never expect____ bluer sky unless we create _____ less polluted world.

With many problems ________, the newly selected president will have a hard time.

May I have a look at _______ book tonight? —Of course, you can. I have ________ old one at home.

I was late for school today ______ I didn’t catch the early bus.