Which of These Correctly Explains the Relationship Between Available to Promise

Which of these correctly explains the relationship between available to promise inventory and the master production schedule for any week beyond the first week of the schedule when production is scheduled? Note that ATP is available to promise, MPS is master production schedule, EI is ending inventory, and OB is orders booked.

A) ATP = MPS + $\sum O B$
B) MPS = ATP + $\sum O B$
C) ATP = EI + MPS - $\sum O B$
D) MPS = $\sum O B$ + ATP - EI