Kay signs a one$-$year lease for a studio apartment with landlord Al$.$ Which of the following is an example of a novation?

A$.$ A balcony unit opens up$,$ so Al substitutes a new lease for that unit, and Kay moves.

B$.$ Kay's job is transferred out the state, so she needs to move; her sister, Kara is looking for a place, so Kara signs an agreement to pay the rent each month, moves in$,$ and takes over payments for the lease.

C$.$ Kay decides she would like to move prior to the one year mark, and Al agrees to end the lease early.

D$.$ Kay's lease ends, and Al lets her remain in the apartment month$-$to$-$month without a new signed agreement.