下列对于文化常识的表述，正确的一项是

Low-carbon lifestyle is of great benefit to improve the world environment, but the problem is difficult to _________．

Rock-climbing is a breathtaking activity, very popular nationwide among young people, ______ cooperation as well as courage plays an important role.

When can he finish the work—-.

Don’t give up and _______learning. I’m sure you will succeed one day.