$11.$ Which property of tool materials helps in retaining sharp cutting edges? a$)$ Toughness b$)$ Ductility c$)$ Hardness d$)$ Brittleness$12.$ The cutting tool material with the highest thermal conductivity is: a$)$ High$-$speed steel b$)$ Carbide c$)$ Ceramics d$)$ Cubic boron nitride$13.$ What characteristic of cutting tool materials helps in resisting chemical wear? a$)$ Toughness b$)$ Hardness c$)$ Thermal conductivity d$)$ Ductility$14.$ Which tool angle is primarily responsible for controlling the depth of cut? a$)$ Relief angle b$)$ Rake angle c$)$ Clearance angle d$)$ Nose radius$15.$ The primary function of chip breakers is to: a$)$ Prevent chips from sticking to the workpiece b$)$ Control the size and shape of chips c$)$ Reduce cutting forces d$)$ Improve surface finish