第21题: [单项选择]Recent research into aging suggests that the body's defense mechanisms may lose the ability to distinguish what is alien. A. insane B. infectious C. foreign D. poisonous 参考答案:C 答案解析:insane患精神病的,极度愚蠢的;infectious有传染性的,易感染的;foreign外国的,异质的;alien外来的,不同的。