More 修饰副词，形容词做比较级为什么后面要跟than?

Question

Gmat OG 2015版
诊断测试 语法39题

用下面五项来替代划线部分

According to scholars, the earlist writing was probably not a direct rendering of speech, but （开始划线）was more likely to begin as(划线结束) a seperate and distinct symbolic system of communication, and only later merged with spoken language.

A. was more likely to begin as
B. more than likely began as
C. more than likely beginning from
D. it was more than likely begun from
E. it was more likely that it began

答案是B。 答案解释上说是这样的

意思是所probably no.........but more than likely 是固定结构的意思吗？
后面那句为什么说more接形容词和副词比较级的时候还要加than?

拜托大神现身讲解一下，不胜感激。

Answer 1

因为这是固定搭配

Answer 2

more than 比什么较多追问

为啥答案里说more修饰形容词副词还要加个than呢？