A shopkeeper sells an item at a 20 % discount on the marked price and still makes a 25 % profit. If the marked price is 500 rupees, what is the cost price of the item?
Arrange the following in decreasing order of number of molecules contained in:
(A) 16 g of \( O_2 \)
(B) 16 g of \( CO_2 \)
(C) 16 g of \( CO \)
(D) 16 g of \( H_2 \)