【单选题】
电流由长直导线流入一电阻均匀分布的金属矩形框架，再从长直导线流出，设图中O<sub>1</sub>，O<sub>2</sub>，O<sub>3</sub>处的磁感应强度为<img src="https://nimg.ppkao.com/2019-04/wuacheng/2019043016241383914.jpg?sign=0296538f895f5b97256110a9efdf12e1&t=62d5d03e" />则（）<br /> <img src="https://nimg.ppkao.com/2019-04/wuacheng/2019043016254418306.jpg?sign=336393289d8bc451d565dbbb49eea13f&t=62d5d03e" /><br /> <img src="https://nimg.ppkao.com/2019-04/wuacheng/201904301625082027.jpg?sign=67de9b0d0edd28a60a1bdf5299afd05d&t=62d5d03e" />
A、A
B、B
C、C
D、D
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