how to turn off chroma(ultra) key of specific area?

Here is another way:

Place your clip with greenscreen on the timeline above your background:

Apply the ultrakey, use an inverted mask to protect area B:

Holding down the Option (Alt) key, click down on the greenscreen clip on the timeline and drag it up a track, this will make an identical copy directly above the original. On the upper clip you just created, remove the Ultra key effect:

Now use an Opacity Mask on the upper clip to protect area C:

If you need to transition to or from this effect, then Nest the clips.

MtD

Alas two or more inverted masks dont work on a single clip.

You need to be creative with one mask:

Multiple inverted masks don't work. - Adobe Support Community