从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
1. In recent years ____global warming is becoming ____concern of people all over the world.
A. the, the B. /, a C. /, / D. the, /
2. This problem may lead to more serious ones if ____ unsolved.
A. left B. remained C. keeping D. making
3. Clothing made of man-made materials has certain advantages over _____ made of natural ones like cotton, wool or silk.
A. one B. what C. that D. the ones
4. The director of our department is determined to promote ______ he thinks is energetic, wise and capable to the position.
