Let xt be a variable denoting the amount of cash in my checking account at the end of month t. That is, xt xt1 represents the change in my checking account balance from the end of month t 1 to the end of month t. Now, let zt > 0 be a variable denoting the total amount of cash I deposit to my checking account in month t; and let yt > 0 be a variable denoting the total amount of cash I withdraw from my checking account in month t. Which constraint appropriately connects xt , xt1, yt , and zt?

(a) xt1 xt = yt + zt

(b) xt1 = xt yt + zt

(c) xt1 xt = yt zt

(d) xt = xt1 + zt yt