2-7-1、如图所示的周期性矩形脉冲信号的傅里叶级数展开式为（）。<img src="https://static.zxtiku.com/wangke/imgs/topic/dfd1840643498c131d04416259d77cf4.jpg"/> A. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/768dbf373c12047845e7053d61224dd1.jpg"/> B. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/c89d339c54a32ae11d1accf98fc3f114.jpg"/> C. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/8ba24b21bd02854e9e2b278a90c391a3.jpg"/> D. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/788f9de7a6933f1ed0a4c99c04197ed1.jpg"/>