New Super Mario Bros./World 7

To get to this world you need to beet the boss of world 5 as mini Mario.

World 7-1
The secret to the second Star Coin is to ride a certain flying ? block to the right instead of taking the 1-Up Mushroom from that block. The block will take you to an area with the Star Coin, many coins, and another 1-Up Mushroom! The bad thing is that when you exit the area, you will have missed the checkpoint, thus to keep your Star Coin you will need to clear the course without dieing.