In Boolean algebra `A + B = Y` implies that :
A. sum of `A` and `B` is `Y`
B. `Y` exists when `A` exists or `B` exists or both `A` and `B` exist
C. `Y` exists only when `A` and `B` both exist
D. `Y` exists when `A`or `B` exist but not when both `A` and `B` exist.
