恒定磁场的能量密度为（）。 A. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/871c2d9086570f44e10c79f7e112fb0e.jpg"/> B. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/b7531a06f6f21c1140a8c7c09819ad83.jpg"/> C. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/e81e3d76b291facc4b2e7befbc2e691e.jpg"/> D. <br/> <img src="https://static.zxtiku.com/wangke/imgs/topic/fa28410f78f3a0fd7f4f68eb4d1629fb.jpg"/>