Which of the following factors does the agricultural tractor's tyres grip depend on?

The grip of any vehicle's tires, including an agricultural tractor's, is directly dependent on the surface it is travelling on. The picture shows an asphalt road whose condition is not uniform – there are wet patches on it, and there is melting snow on the verge. Each of these elements (dry asphalt, a wet surface, slush) provides a different, and usually lower, level of grip, which confirms that both the type and condition of the road surface are crucial.