题目

       It was another rainy day, it had stopped raining but the ground was still wet, with this remarkable smell of freshness, I was in the car with my friend, and ahead of us was a pitiful-looking old man, with a walking stick in his hand as it seemed he was having a hard time standing up or barely doing any effort. I have always had a weakness for old people; they break my heart, especially the ones begging on the streets, or selling flowers, I feel sad for them, having to be on the streets instead of being taken care of by their families or the government.        There w地球上最大的生态系统是（ ）A.草原生态系统B.生物圈C.海洋生态系统D.森林生态系统