deer population is being struke down by the disease

现在进行时的被动语态
http://media.open.edu.cn/media_file/rm/ip3/liuyf/2006_11_08/yyII(1)/unit%2011/B122/html/text02.html

当然是：“现在进行时的被动语态 ”

现在进行时（is being ）+被动语态（be struke原型stroke打击的意思）

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IM SO PLEASED IF MY ANSWER MAY HELPED U TO SOLVE SUCH PROBLEM，BUT ITS JUS 4 A RE.
BLESSING U

被动进行时