大家帮忙看看我这个题

这是主语从句的问题。
That (nearly all behavior is learned behavior) is basic assumption of social scientists.
that 是指代nearly all behavior is learned behavior这句话的
如果选A， 那么分析句子成分，这句话就少了主语。
选B，这句话除去括号中的句子，仍然是完整的。

That nearly (<that省了>all behavior is learned behavior ) is basic assumption of social scientists

去了That，句子没主语了。

it是形式主语

去了That，句子没主语了。

that 引导主语从句。。。