Friction bearings are:

Friction bearings, also known as plain bearings or sleeve bearings, are typically made from materials that are softer than the shaft they support. This design choice allows the bearing material to take on the wear and tear associated with motion. When the softer bearing material wears down, it can be easier and more cost-effective to replace the bearing instead of the shaft, which is designed to have higher durability.

Using a softer material also helps to accommodate slight misalignments and provides a better surface for lubrication retention, which is critical in reducing friction, transferring loads, and enabling smooth rotation. The softer bearing material can deform slightly to create a better fit, reducing stress concentrations on the shaft, which could lead to failure.

While the other choices introduce various attributes of bearings, they do not align with the principle of friction bearings' material properties and their typical application. For instance, a hard bearing material would not effectively protect the shaft from wear, and a bearing that covers two-thirds of the shaft or is located only at the ends would not typically provide the support necessary through the entire length as required in many applications.