在磁感强度为<img src="https://nimg.ppkao.com/2019-04/wangjue/2019042816584490294.jpg?sign=325c2158e5d525cf4f67191c333022be&t=62d710c6" />的均匀磁场中作一半径为r的半球面S,S边线所在平面的法线方向单位矢量<img src="https://nimg.ppkao.com/2019-04/wangjue/2019042816590365434.jpg?sign=bdec30f0524244663ed2984dece2b68f&t=62d710c6" />与<img src="https://nimg.ppkao.com/2019-04/wangjue/2019042816584490294.jpg?sign=325c2158e5d525cf4f67191c333022be&t=62d710c6" />的夹角为a,则通过半球面S的磁通量(取弯面向外为正)为()<br /> <img src="https://nimg.ppkao.com/2019-04/wangjue/2019042816592230478.jpg?sign=b4c2d40387bca72c831a75ce2053d639&t=62d710c6" />
A、πr2B
B、2πr2B
C、-πr2Bsina
D、-πr2Bcosa