几道英语填空题,请英语高手献身~

几道英语填空题,请英语高手献身~
1.When the police arrived there, the thief had e_______.
2.The c_______ have never reached the top of the mountain.
3.The i_______ population may be the greatest challenge of the world.
4.The s_______ of the houses in the south are quite different from those in the north.