He is so busy. He cannot afford enough time with his son ______ he wants to.
We can never expect bluer sky unless we create less polluted world.
—rapid progress they have made! — Right. And it is true the students from other developing countries.
The plane is flying ____ a speed of 500 kilometers an hour ____ the way to Kunming.
You can’t learn to play the piano without practising.____, you can’t learn a language well without using it.
We’ll ____ three groups to carry out the experiment in chemistry.