请教一道语法

6．Although the bite of brown recluse spiders are rarely fatal, they cause chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its poison.
(A) brown recluse spiders are rarely fatal, they cause chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its
(B) brown recluse spiders are rarely fatal, they cause chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to their
(C) the brown recluse spider is rarely fatal,it causes chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to their
(D) the brown recluse spider is rarely fatal, it causes chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its
(E) the brown recluse spider is rarely fatal, they cause chronic

答案是D，不明白为什么最后到用its，不用their，因为应该是蜘蛛的毒，而不是bite的毒呀

(D) the brown recluse spider is rarely fatal, it causes chronic flesh wounds, posing the greatest danger to the infant and elderly, who are particularly vulnerable to its

啊，，，，，知道啦