Final answer:
Lower do, or Middle C, in the Key of C Major appears on the first ledger line below the staff, serving as a central reference point for musicians on the piano.
Step-by-step explanation:
In the Key of C Major, lower do, which is also known as Middle C in music theory, can be found on the first ledger line below the staff. When looking at a grand staff, which includes both the treble and bass clefs, Middle C will be on the first ledger line above the bass staff or the first ledger line below the treble staff. This note is a pivotal reference point for musicians as it's centrally located on the piano and serves as a guide for where other notes are placed in relation to it.