Choose the best topic sentence from the group below by writing A, B, C or D.

单项选择题Read the following paragraph carefully and choose the best topic sentence from the four possible answers that follow the paragraph by writing A, B, C or DTopic Sentence: ______At the turn of the century we re faced with many social and economical problems, of which environmental pollution might be the most critical and vital. Every day when we look up, the sky is sadly gray. The moment we walk on the pavement or ride a bike in the street, we will smell the strong, thick gas emitted out of cars and trucks, along with much terrible noise. Each time we turn on TV, a couple of bad news about global warming and acid rains will flow into our ears. Our earth, as the single homeland we are able to count on now, is sufficiently eroded by air and land pollution.