$571.$ The 'transition temperature' for ductile to brittle behaviour of steel increases with increase in the $\_\_\_\_\_\_\_\_\_\_$ content in steel.

Carbon

Manganese

Both $\text{'}$a$\text{'}$ nor $\text{'}$b$\text{'}$

Neither $\text{'}$a$\text{'}$ nor $\text{'}$b$\text{'}$