Left pan of a faculty 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. $\frac{{100}}{{11}}$$%
• B. $\frac{{200}}{{11}}$$%
• C. $\frac{{100}}{{9}}$$%
• D. $\frac{{200}}{{9}}$$%