关于一句英语句子的疑惑:"The main reason is ____that+从句",空内是填obvious还是obviously?

问题描述:

关于一句英语句子的疑惑:"The main reason is ____that+从句",空内是填obvious还是obviously?
答案就是从obvious和obviously里面选择,并给下原因,

The main reason is obviously that
显而易见,这最主要的原因是你的答案应该是对的,能给解释一下么?为什么obvious不可以,它作表语难道不对么?