设两个独立随机变量x,y的分布规律为<img src="https://static.zxtiku.com/wangke/imgs/topic/b2ddc684d281901dd889f7c467759c6f.jpg"/> <img src="https://static.zxtiku.com/wangke/imgs/topic/d8d78b9a0f9d2d7cc77cf80a8fcfe732.jpg"/> 则z=x+y的分布律（） A. <img src="https://static.zxtiku.com/wangke/imgs/topic/4b87d77f122d774326f62b42eb22025c.jpg"/> B. <img src="https://static.zxtiku.com/wangke/imgs/topic/90c19c39b153fb529a17f12e8c065277.jpg"/> C. <img src="https://static.zxtiku.com/wangke/imgs/topic/f61c648a3034b0a791135d9e3b798b6e.jpg"/> D. <img src="https://static.zxtiku.com/wangke/imgs/topic/ae448eb2b99668028979cb8ed9aba2fa.jpg"/>