| 消除噪声污染是当前环境保护的一个重要课题．内燃机、通风机等在排放各种高速气流的过程中都发出噪声，干涉型消声器可以用来消弱高速气流产生的噪声，干涉型消声器的结构及气流运行如图所示，波长为λ的声波沿水平管道自左向右传播，当声波到达a处时，分成两束相干波，它们分别通过r1和r2的路程，再在b处相遇，即可达到消弱噪声的目的．若△r=|r2-r1|，则△r等于（ ） A．波长λ的整数倍 B．波长λ的奇数倍 C．半波长λ/2的奇数倍 D．半波长λ/2的偶数倍  |
答案：C
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| Dear friend, My name is Alice Green. I am 1___________ American girl. I'm thirteen. I have two brothers. One is Jack, 2___________ is Tom. We go 3___________ school five days a week. We stay at home 4___________ Saturdays and Sundays. We are in 5___________ same school. My father is a worker. My mother is a teacher. Mother often 6___________ China is great and Chinese food is good. The Chinese people 7___________ very friendly. I like spring. My brothers like 8___________. We 9___________ like watching TV. What about you? Let us 10___________. Please write to me soon. Yours, Alice
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