请问新通用大学英语综合教程第三册 unit2 text c 第四段

请问新通用大学英语综合教程第三册 unit2 text c 第四段
Hospital patients are predominantly the elderly and the very young,two categories least likely to have AIDS.Why does Mr.Bennett propose to look there for the virus instead of among high-risk groups?Because hospitals are where testing is easiest.So too argued the drunk driver who lost his keys in the dark and explained he was searching for them unde r the lamppost because the light was better.
还有 这段话有哪些主要的语法