一道英语单选题,请解析

题目

一道英语单选题,请解析
According to the research,was once considered rare and healthy food is no more than something ordinary.
A．that B．whether C．all D．what

答案

According to the research,was once considered rare and healthy food is no more than something ordinary.
A．that B．whether C．all D．what
选D.　what was once considered rare and healthy food 是主语从句,因为从句中缺少consider的宾语,所以用what来引导.what在从句中当（被动语态）主语.