Suppose a school has three periods (called 1, 2, and 3) during which a class can be scheduled. For each class, there are three variables. For example, for class A, there are variables XA1, XA2, and XA3. Setting XA2 = 1 represent scheduling class A during period 2. a. give a boolean expression that evaluates to 1 if and only if a is scheduled in at least one of the three periods.