If a Company Has a Demand of 1,600 Units for the Month

If a company has a demand of 1,600 units for the month of May and a beginning of 900 units, and if X_m = production in May and L_m = the number of lost sales in May, which of the following equations represents the material-balance constraint?

A) X_m - I_m + 900 + L_m = 1,600
B) X_m + I_m - 900 + L_m = 1,600
C) X_m - I_m + 900 - L_m = 1,600
D) -X_m - I_m + 900 - L_m = 1,600