A shopkeeper reduces the price of a shirt by ₹15 every week. The prices over four consecutive weeks are ₹585, ₹570, ₹555 and ₹540. Choose the option that correctly states whether these prices form an arithmetic progression and the common difference.
a) Yes, common difference ₹−15
b) Yes, common difference ₹15
c) No, common difference ₹−15
d) Yes, common difference ₹−30