ANS:B - coarser

For grinding softer materials, the grinding wheel should have a coarser grain size. Here's an explanation:

1. Grain Size: The grain size of a grinding wheel refers to the size of the abrasive grains bonded together to form the wheel. It is indicated by a grit number, where a higher number represents finer grains and a lower number represents coarser grains.
2. Grinding Soft Materials: When grinding softer materials, such as aluminum or soft steel, a coarser grain size is preferred. This is because softer materials tend to clog finer abrasive grains more quickly, reducing the efficiency of the grinding process and leading to overheating of the workpiece. Coarser grains allow for more efficient chip removal and cooler grinding, resulting in better surface finishes and longer wheel life.
3. Choice of Grain Size: The choice of grain size depends on the material being ground, the desired surface finish, and the grinding conditions. For softer materials, a coarser grain size is generally more suitable, while for harder materials, a finer grain size may be required to achieve the desired finish and accuracy.