How do you get an A on sonic heroes

You must complete the stage you are on fast, get lots of rings, get those little balls of power aka red,yellow, and blue and that's it. Another thing when you break a metal box you will find a key collect them and don't let any enemies hit you. When you complete the stage and you have the key you might get a chaos emerald. After you get all of them go to the right and you might see METAL SONIC.HARD