下列结构中不属于器官的是（ ）
A：白菜叶
B：洋葱表皮
C：西瓜
D：人体的胃
His methods are regarded as the of working by his leaders and workmates.
用数学归纳法证明“1＋2＋22＋…＋2n－1＝2n－1(n∈N＋)”的过程中，第二步n＝k时等式成立，则当n＝k＋1时，应得到( )
