Task: Question Answering
Input (English): Question: If a farmer sells 15 of his chickens, his stock of feed will last for 4 more days than planned, but if he buys 10 more chickens, he will run out of feed 6 days earlier than planned. If no chickens are sold or bought, the farmer will be exactly on schedule. How many chickens does the farmer have?

 Option A: 20

 Option B: 50

 Option C: 40

 Option D: 30

 Option E: 60
Output (English): Option D