设随机变量<img src="https://static.zxtiku.com/wangke/imgs/topic/3dab0dfb141c49162ef8b2c1ae0c4dfc.jpg"/>的方差存在，则（）. A. <img src="https://static.zxtiku.com/wangke/imgs/topic/8b58cee961d2703ff4b7867435c21caa.jpg"/>； B. <img src="https://static.zxtiku.com/wangke/imgs/topic/d2bdce60332678c39534812fda796649.jpg"/>； C. <img src="https://static.zxtiku.com/wangke/imgs/topic/51a7edd66086be1bdd1a5ae8bdc636f6.jpg"/>； D. <img src="https://static.zxtiku.com/wangke/imgs/topic/bc9a29422b3f9c92da40f6c95b07d637.jpg"/>.