B
题干:小鸟不高兴,因为____。
A.狮子经常虐待她
B.她认为自己没用
C.她想杀死狮子
D.她并不比猫头鹰聪明
根据第五段"I'm so small, weak and useless (无用的)," she cried. "I want to be big and strong likethe lion, He is very important, but I'm not."(“我又小又弱又没用,”她哭着说。“我想变得像狮子一样又大又壮。他很重要,但我不是。”)可知,这只小鸟不开心是因为她认为自己弱小且无用,故选B。