Accidentals cannot be used when there is a key signature true or false.

False; for example, if we're in the key signature of G major which features the notes G, A, B, C, D, E and F#, any use of an F natural note would be considered to be an accidental.

the correct answer would be "the amiens cathedral"

the answer is counterpoint.

counterpoint definition: "the art or technique of playing a melody or melodies in conjunction with another, according to fixed rules."

counterpoint is present in a lot of baroque music- such as bach (notably his wtc prelude and fugues) and scarlatti, but can also be found in music by mozart, beethoven. it can even be found in music as modern as john williams!