With the word“PM2．5"_____ appearing in media reports，people pay greater attention to it and seek health tips for smoggy days．

It is the prevention of disease______its successful treatment that has led to the rapid increase of the world’s population.

Travelling west, you set your clock ______; travelling east, you set it ahead.

 — Harry is too thin for his age.— So he is, but ______ he is very healthy.

As is often pointed out, knowledge is a two-edged weapon which can be used ________ for good or evil．