whats the nuances of interjections え？ は？ そうか ? and 何だ？


I know that interjections え？ は？ そうか ? and 何だ？is used when we are in doubt. and
i would like to know whats the nuances between those word. here i give the
examples....

1st:

> ルフィ：おれは死んでもいいんだ！  
>  コビー： **え？**

2nd:

> ヘルメッポ：おいこのガキ投げすてろ！！  
>  海兵： **は？**

3rd:

> ゾロ：おれはおれの...やりたい事を成し遂げる！！！  
>  ルフィ：ふーん、 **そうか** 。でも おれなら一週間で餓死する自信あるけどね。

4th:

> ゾロ：仕方んねエからその辺の海賊船を狙って。生活費を稼いでた. . . それだけだ  
>  ルフィ： **なんだ** ？お前迷子か。

Can those word being substitute to one and another?




They are kinds of injections.

え？ - It expresses surprised. Used when a speaker was surprised. Anyway, "what"
in English.

は？ - It also expresses surprised, but it expresses not agreeing a opponent's
an opinion.

そうか。 - In English, "is that so?"

なんだ？ - It's hard to explain for me, because I don't almost use the usage like
that. Its usage, "what?"

