at the convenience store, magazines cost x dollars each, sodas cost y dollars each, and candy bars cost z dollars each. the amount of money martha had left after purchasing items from the convenience store is represented by the following expression. $21.45 - (ay bx) which of the following is the best interpretation of a? a. the number of sodas martha purchased at the convenience store. b. the number of magazines martha purchased at the convenience store. c. the number of sodas and magazines martha purchased at the convenience store. d. the number of candy bars martha purchased at the convenience store.