An engineer in a steel company hardens gears made of carbon steel by quench. However, the thickness of the hardened surface layer is not thick enough. The engineer proposes to add a small amount of alloying elements to the steel in order to increase the thickness of hardened layer. There are three choices: a) adding 1%Cr, b) adding 1%Ni, and c) adding 0.5%Cr and 1%Ni. Which choice will lead to the largest increase in thickness of the hardened layer and which one will lead to the least increase in the thickness? Explain your answers