阅读下面短文，掌握其大意，然后从各题所给的四个选项中选出可以填入空白处的最佳选项。

With the rapid development of modern industry, the environment around us is becoming worse and worse. Rivers that used to be clean are now seriously ___. The air we breathe every day is filled with harmful gases. It is high time that we ___ action to protect our only home. First of all, we should learn to sort the rubbish before throwing it ___(3). Besides, we ought to use public transport more often instead of driving private cars. Small steps like these can make a big ___ to our planet. After all, we have only one earth to ___.

【第3空】