Complete the following sentence with a (compound) word derived from the one(s) given in bracket.
I don't know if he did it through ignorance or just plain （ ）.(stupid)
【正确答案】：STUPIDITY
【题目解析】：
本题考查的是常用形容词的名词形式。
由plain(此处作形容词：纯粹的)可知，空缺处应填名词，形容词修饰名词，stupid的名词形式是stupidity意为“愚蠢”。
句意：我不知道他是因为无知还是纯粹愚蠢才做的这件事。