下列有关生态系统的主要功能的叙述，正确的是（  ）

   “Everything happens for the best,” my mother said whenever things weren't going my way. “Don't worry, one day your luck will change.” Mother was right, as I discovered after I had finished my college education. I had decided to find a job in radio. One day, I wanted to host(主持) a sports program. I went to Chicago and knocked on the door of every station. But I got turned down every time.
     In one station, a kind lady said my problem was that I hadn't got enough experience. “ Get some work with a small station and work your way up,” she said.
     I went back home. I couldn't get a job there, either. Then my dad told me a businessman had opened a store and needed someone to help him. But again, I didn't get the job.
     I felt really down. “ Your luck will change,” Mom said to me. Dad lent me the car to help me to look for my job. I tried another radio station in Iowa. But the owner, a nice man, told me he had already had someone.
     As I left his office, I asked, “How can someone be a sports announcer(播音员) if he can't get a job in a radio station?”
I was waiting for the lift when I heard the man call. “ What did you mean? Do you know anything about football?” He put me in front of a microphone and asked me to try to imagine that I was giving my opinion on a football game, I succeeded.
     On my way home, Mom's words came back to me, “One day your luck will change, Son. And when it happens, it'll feel doubly(加倍的) good because of all the hard work you've had.” At that moment I knew what just what she meant.

    I had just arrived in this Asian country for a one-year teaching position. One day, I took the subway to visit some ancient palaces and temples in the downtown. The following account of what happened to me has taught me much about culture 1.

    Since all the 2 were taken, I stood. Suddenly, I felt someone pulling on my bag. 3 I probably was in someone's way, I moved over slightly. But in one quick motion(动作), I felt my bag removed from my back, and in a flash it was 4. I turned around to see who the thief was. I looked at the people standing behind me, but didn't see my bag or any 5. My heart sank and I began to 6.

    I glanced around the car only to find directly across from me was an elderly lady, and sitting on her lap was my 7. I tried to get it back from her lap. But as I began to 8 it up, she quickly grabbed(抓住) it back and held onto it. I looked around at the people standing beside me, and those sitting beside her, but no one took any 9 of the situation. Trying not to cause a(an) 10, I tried to negotiate through gestures. I used my hands as best as I could, but she 11 my requests for my bag and pointed to my back. She picked up my bag, showing how 12 it was. I finally began to understand. She was holding my bag to13 me.

    At the next stop, a middle-aged woman got on the crowded subway. Another elderly woman sitting down took her bag, 14 it on her lap. They didn't talk; 15 this older woman was more than pleased to sit with this stranger's bag on her lap throughout her journey.

    As the subway pulled into the main downtown station and I was getting ready to get off, the woman 16 handed me back my bag. But 17 I had a chance to thank her, she had disappeared into the crowd.

    Sadly, this considerate custom was more 18 to me than if I had been robbed. Everyone back home had heard of being robbed—that was 19 city behavior—but having a stranger hold onto someone's bag out of 20, in a city of twelve million people—that was truly unusual.