在水平桌面上，一个圆形金属框置于匀强磁场B1中，线框平面与磁场垂直，圆形金属框与一个水平的平行金属导轨相连接，导轨上放置一根导体棒ab，导体棒与导轨接触良好，导体棒处于另一匀强磁场B2中，该磁场的磁感应强度恒定，方向垂直导轨平面向下，如图甲所示，磁感应强度B1随时间t的变化关系如图乙所示，0～1.0s内磁场方向垂直线框平面向下。若导体棒始终保持静止，并设向右为静摩擦力的正方向，则导体棒所受的静摩擦力f随时间变化的图象是选项图中的
A.
B.
C.
D.
答案：D
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| Back in the 15th century, in a tiny village in Germany, lived a family with eighteen children. Eighteen! In order 1 to keep food on the table, the father, a goldsmith(金匠) by 2 , worked almost eighteen hours a day at his trade. Despite their apparently 3 condition, two of the eldest children had a dream. They both wanted to pursue their talent for art, 4 they knew well that their father would never be able to 5 either of them to study at the Academy. After many long discussions at night in their 6 bed, the two boys finally worked out an agreement. They would toss (掷) a coin. The 7 would go down into the nearby mines and, with his earnings, 8 his winning brother for the academy. Then, in four years, he would support the other one. Then Albrecht Durer won the toss and 9 to Nuremberg. Albert went down into the dangerous mines and, for the next four years, 10 his brother, whose work at the academy was almost an immediate sensation(轰动). By the time he graduated, he was beginning to 11 considerable fees for himself. When the young artist 12 home and said to his brother, "And now, Albert, blessed brother of mine, now it is your 13 . Now you can go to the Academy to pursue your dream, and I will 14 you. " Albert rose and wiped the 15 from his cheeks. "No, brother. I cannot go to Nuremberg. It is too 16 for me. Look…look what four years in the mines have done to my 17 ! I cannot even hold a glass, much less make delicate lines on canvas(画布) 18 a pen or a brush. " More than 450 years have 19 . By now, among Albrecht Durer's hundreds of masterful portraits, "The Praying Hands" is one 20 creation that can catch the world's hearts. | ||||
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