Friction factor for a hydraulically smooth pipe at $N_{R_e}=2100$ is $f_1.$ If the pipe is further smoothened $($i$.$e$,$ roughness is reduced$),$ the friction factor at the same value of $N_{R_e},$ will

A$.$ increase.

B$.$ decrease

C$.$ remain unchanged.

D$.$ increase or decrease depending on the pipe material.