1. ________ we don't stop climate change, many animals and plants in the world will be gone.

   A. Although            B. While          C. If             D. Until

2. —Peter, please send us postcards ________ we'll know where you have visited.—No problem.

A. but                 B. or             C. for            D. so

3. ________ birds use their feathers for flight, some of their feathers are for other purposes.

   A. Once                B. If             C. Although       D. Because

4. If you don't understand something, you may research, study, and talk to other people ________ you figure it out.

   A. because             B. though         C. until          D. since