Left pan of a faulty balance weighs 100 grams more than its right pan. A shopkeeper keeps the weight measure in the left pan while buying goods but keeps it in the right pan while selling his goods. He uses only 1 kg weight measure. If he sells his goods at the listed cost price, what is his gain? a. `(100)/(11)%` b. `(200)/(11)%` c. `(100)/9%` d. `(200)/9%`