设随机变量X与Y独立，且<img src="https://static.zxtiku.com/wangke/imgs/topic/32ae835becceffa76013154eff837ea5.jpg"/>，<img src="https://static.zxtiku.com/wangke/imgs/topic/d816b5fa395d180f6d03844d22a17c26.jpg"/>. 令 <img src="https://static.zxtiku.com/wangke/imgs/topic/3a27ef5f47de65bdf086bd5fc2dc1610.jpg"/>. 要使X与Z独立，则p的值为（）.