Still confused about wa and ga

I'm still confused about the difference between wa and ga in interrogative sentence.

I found this short conversation:
A: taisetsu na hito wa dare?
B: Jimu ga taisetsu.

Why does it use wa instead of ga? Why doesn't it be like this:
A: taisetsu na hito ga dare?
B: Jimu ga taisetsu.

Thank you in advance :)

The question by A can be either:

A: Dare ga taisetsu na hito?
or
A: Taisetsu na hito wa dare?

...but not the second one you've listed. Please search this forum questions, there have been many attempts to explain this :)

It is not that difficult.
When the speaker introduces a new topic or thinks that the following information is new/ unknown for the listener it is used ga instead of wa.

Moreover, when the speaker following talks about the same topic and/or concernes on the topic it goes on with using wa.

Were you able to understand?

I got this from this site:
Wa or Ga ??
Both are correct. The difference is:
If I say: Watashi wa Mark desu. It means that the information I want to highlight is after the "wa".
If I say: Watashi ga Mark desu. It means the information I want to highlight is before the "ga".
Here are two examples:
If someone asks me "What is your name?" and if someone else asks "Who is Mark?" In English, the answer to both is "I am Mark". But in Japanese the answer to "What is your name?" is: Watashi wa Mark desu. Because "Mark" is the information I want to give.
For the question: "Who is Mark?" for example when a teacher is asking a group of students, then I will answer: "Watashi ga Mark desu". Because "Mark" is already a given, meaning that everyone knows there is a "Mark" in the room, the unknown information is who. So in the answer, I want to highlight "I", therefore I use "ga" to indicate that it is the information I want to focus on.
I hope you can understand what I am trying to say.

Thank you for everyone. :)

But I'm a little confused at Okami's explanation.