题目

阅读理解 Mrs. Jackson is an American doctor. She is now in China. She works in a children’s hospital in Chongqing. She is also learning Chinese medicine there. She likes Chinese medicine very much. She loves to work for children. She works hard in the day and reads English books on Chinese medicine at night. She learns Chinese from the Chinese doctors and her Chinese friends. Now she can speak some Chinese. She can read some Chinese, too.    Her husband, Mr. Jackson, is a teacher. He teaches English in a middle school in Chongqing. He works hard, too. He works from Monday to Fr 由于流体具有粘滞性，因而物体在流体中运动要受到流体的阻力．科学家们已测得半径为R的球在流体中以速度v 运动时受流体阻力大小为F=6πηRv，其中η为流体的黏滞系数，不同流体η不同，它由流体的性质决定．某课外兴趣小组在研究性学习活动中为了测定某种流体的黏滞系数，设计如下实验：让密度为ρ=1.2×103 kg/m3半径为R=3cm的实心球在密度为ρ0=0.8×103 kg/m3的透明液体中竖直下落，发现球先加速下落后匀速下落，该兴趣小组的同学用频闪摄影的方法测出球匀速下落时的速度为v=4.9m/s．若球的体积计算式为V=4πR3/3，g=9.8N/kg．则该流体的黏滞系数η为（　　） A． 0.16N•s/m      B．  0.48N•s/m      C． 0.16N•s/m2    D． 0.48N•s/m2