椭圆<img src="https://zk.995w.com/lqwvjeImg/8a/8a016cbca0cf2a67a50255ba083e8b92.png"style="width:110px;height:54px;">的离心率为<img src="https://zk.995w.com/lqwvjeImg/87/87c6503607208d5df80f5ba6f7ef1a8e.png"style="width:21px;height:56px;">,则k的值为( )<br>·-21<br>·21<br>·<img src="https://zk.995w.com/lqwvjeImg/1f/1feccc775155ed3c78b26c1b8e109f36.png"style="width:82px;height:56px;"><br>·<img src="https://zk.995w.com/lqwvjeImg/b7/b76900c68e1652660bdae61f497bfdec.png"style="width:66px;height:54px;">