根据短文意思和所给首字母,写出正确的单词,使句子完整、正确.

    Water and its i  1  to human life were the centre of the world's attention last week. March 22nd was

World Water Day and had the theme "Water for Life". There are more than one billion people in the

world who live w   2  safe drinking water. The United Nations h   3   to cut this number in half by 2015.

Solving s   4   a big problem seems like an unreal challenge(挑战). But e   5   ,even teenagers, can do

something to help.