如图所示，一长为a，宽为b的矩形线圈放在磁场<img src="https://nimg.ppkao.com/2019-04/wuacheng/2019043017000714246.jpg?sign=960b99fc7042b473023ed76cadfb982d&t=62d60196" />中，磁场变化规律为<img src="https://nimg.ppkao.com/2019-04/wuacheng/2019043017001955590.jpg?sign=34a4831aba03945ed5b4ea148fcf7fb9&t=62d60196" />线圈平面与磁场垂直，则线圈内感应电动势大小为（）<br /> <img src="https://nimg.ppkao.com/2019-04/wuacheng/2019043017002928717.jpg?sign=d49b8923d7e190f486d0e71c56bf97ea&t=62d60196" /><br /> <img src="https://nimg.ppkao.com/2019-04/wuacheng/2019043017003736460.jpg?sign=07b1e49b605a31c5515c71b35dfcc1bd&t=62d60196" />