W-TC's E flat M and not D sharp M, and,

In W-TC, Book I, Prelude and Fugue No. 7 are in E flat major. Why wouldn't
they be cited as being in D sharp major?

In No. 8, the prelude is in E flat minor while the fugue is in D sharp
minor. Why wouldn't they both be in one or the other key signature?

Can some very patient person here help this neophyte see this from Bach's
point of view?  Thanks very much.
bill carroll, toronto