设X和Y是相互独立的随机变量,且X~Exp(<img src="https://static.zxtiku.com/wangke/imgs/topic/85f0c58223faf1d32aa2aa958437f56b.png"/> ),Y~Exp(<img src="https://static.zxtiku.com/wangke/imgs/topic/c08c693d7767941f00956ce108260ffe.png"/> )。定义<img src="https://static.zxtiku.com/wangke/imgs/topic/4b28622e8aa868e10d9ad062d067c608.png"/> ,则下列叙述正确的有()