A couple who own an appliance store discover that if they advertise a sales discount of 10% on every item in the store, at the end of one month the number of total items sold increases 25%. Their gross income from sales for one month increases by what percent?	Choose the correct choice from the following:  A) 5 %  B) 7.5 %  C) 10 %  D) 12.5 %  E) 15 %

Rationale: "let p be the original price and let x be the number of items sold originally . the original income is p * x . after the changes , the income is 0.9 p * 1.25 x = 1.125 * ( p * x ) , an increase of 12.5 % . the answer is d ."
Answer: D