ANSWER:
c minor
Step-by-step explanation:
In the beginning of the third movement of Beethoven's Piano Sonata in C minor, the main theme is in the key of C minor. You can tell because the notes being played match the pattern and sound of the C minor scale. The key of a piece of music tells you what notes will be used most often and what chords will be used to support the melody. In this case, the main theme starts and ends on the note C, which is the first note of the C minor scale. This gives the music a dark and dramatic feel, which is typical of music written in a minor key.