信元是信元交换的单位。为控制差错，在信元中包括CRC校验和，其生成公式为 (1) ，校验和对 (2) 进行校验。信元交换采用 (3) 技术进行复用。在交换过程中，当实施VP交换时，其中VPI、VCI的变化情况是 (4) 。若在交换过程中出现拥塞，该信息被记录在信元的 (5) 中。
4（）
A．VPI、VCI都不变
B．VPI、VCI根据需要变化
C．VPI不变、VCI根据需要变化
D．VCI不变、VPI根据需要变化
参考答案：C
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| I first met her on the beach near where I live. I drive to this beach a 1 of three or four miles, whenever I'm feeling down. At that time she was 2 something. Looking 3 , she said, "Hello." I nodded,not wanting to 4 with a small child. "I'm Wendy and I'm building something," she said. "I see. But what is it?" I asked, not 5 . "I don't know. I just like the 6 of sand." A sandpiper (矶鹞鸟) flew by. "That's a 7 . Mum says sandpipers come to bring us joy." "Good-bye, joy," I thought to myself, "hello, 8 ." and walked away. My life then seemed completely out of 9 . "Come again," she called, "and have another 10 day." But the days and weeks that followed 11 to other people:naughty boys and a 12 mother. One morning I suddenly realized I needed my 13 and headed for the beach. The breeze was cool, but I walked along,trying to recapture (捕捉) the inner peace I needed. I had almost 14 the child so I was surprised when she appeared. " 16 do you live?" I asked. She pointed toward a summer cottage. Then one day, I rushed to my beach in great pain, even in no__16__to greet the girl. "If you don't mind," I said coldly, "I'd rather be 17 today." She seemed unusually 18 and out of breath. "Why?" she asked. "My mother died!" "Did it hurt when she died?" "Of course it hurt!" I shouted, misunderstanding her. When I next went to the beach, she wasn't there. Feeling 19 , and admitting I 20 her, I went up to the cottage. A woman answered the door. "Wendy, my daughter, died last week. She had leukemia (白血病). Maybe she didn't tell you." | ||||
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