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Start中等

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illustrate：
- The "down", "left", and "right" keys control the direction, while "up" is for transformation;
- Difficulty can be adjusted before game starts and during pause;
- Difficulty is divided into three levels: "easy", "medium" and "hard";
- The corresponding scores are 10, 20, and 40;
- The multiples that can be eliminated simultaneously across multiple rows are 1, 4, 10, and 20;