WhatisknowntousallisthatAmericaisadevelopedcountrybelongingtotheFirstWorld.
Whatisknowntousall是整个句子的主语的话,IS是谓语,那么后面thatAmeriacaisadevelopedcountrybelongingtotheFirstWorld这一整句是表语吗?
是一个主语从句+一个表语从句?