First choose the game difficulty, and then operate the role reasonably, shooting the crow that flies in the sky. How to operate ？ A - GO LEFT D - GO RIGHT W - GO FORWARD S - GO BACKWARD SPACE - SHOOT ESC - PAUSE GAME UP - INCREASE GUNNING ANGLE DOWN - REDUCE GUNNING ANGLE LEFT- ROTATE ROLE TO LEFT RIGHT- ROTATE ROLE TO RIGHT Game difficulty EASY — The birds fly very slowly NORMAL — The birds fly at normal speed HARD - The birds fly very fast