Sara bought both German chocolate and Swiss chocolate for some cakes she was baking. The Swiss chocolate cost $3.5 per pound, and German chocolate cost $2.8 per pound. if the total the she spent on chocolate was $28 and both types of chocolate were purchased in whole number of pounds, how many total pounds of chocolate she purchased?  Select the correct option:
A) 9 B) 8 C) 10 D) 12 E) 15

Rationale: "if there were all the expensive ones , 3.5 . . . . there would be 28 / 3.5 or 8 of them but since 2.8 $ ones are also there , answer has to be > 8 . . if all were 2.8 $ ones , there will be 28 / 2.8 or 10 . . . so only 9 is left answer a . ."
Answer: A