A. 1 mm
B. 1.6 mm
C. 2.5 mm
D. 4 mm
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The entire original tread should be continuous and at least 1mm deep. Don't ride a motorcycle with worn tyres. Your tyres are your only contact with the road so it's very important that you ensure they are in good condition. In the case of worn tyres, it dramatically increases the risk of aquaplaning or hydroplaning on water. This is where the tyre rides up on the surface of the water rather than the tread displacing the water so that the tyre maintains contact with the road.