---The air pollution is terrible.---It will be worse _______ we take action to protect the environment.
We live in a world _______ colours.
Not only the students but also their teacher _______ about the four people.
---Have you found ________ in the grass, Millie? ---No, not yet.
---It’s time to sleep, Daniel. ---Oh, Mum, I won’t go to bed _____ I finish my homework.
I don’t understand ________.