在补体活化的经典途径的激活过程中，不被裂解的组分是（） A.C2 B.C3 C.C4

     It was a cold winter afternoon. Robert stopped for a moment as he crossed the bridge and looked
down at the river below. There were hardly any   1    on the river. Near the bridge, however, almost
direct below,   2    was a small canoe (独木舟), with a boy in it. The boy was wearing  3    and shivering (颤抖) with cold, Robert noticed.     
     Just then he heard a cry, “Help! Help!” The cry   4    from the river. Robert looked down. The boy
was   5    the water and his canoe was floating away. “Help! Help!” he called again.    
      Robert was a good   6    . Taking off his clothes, he   7    into the river.
The   8    water made him tremble（颤抖） all over,    9    in a few seconds he reached the boy. 
“Don’t be afraid,” he said and started to swim towards the river bank,  10   the boy with him. But at
that   11   he noticed a large motor boat under the bridge. There were several people on the boat,
all   12   in his direction. Robert   13   to swim towards the boat.   
     “Give me a hand,” he shouted   14   he got near the boat. He   15   up into a row of faces. “It’s
funny,” he thought. “They look so   16   .” Silently they helped the boy into the
boat and   17   him in a blanket (毯子). But they did not move to   18   Robert.  
      “Aren’t you going to pull me   19   , too?”  
    “You!” said one of the men. Robert noticed that he was standing next to a large   20   . “You! Why,
we were making a film and you spoiled (破坏) a whole afternoon’s work! You can stay in the water!”