$($a$)$ As shown in the figure $($a$)$ below, if the section of the T$-$beam must add longitudinal reinforcement, where should it be arranged? $($Please draw it$4\%),$ and explain the benefits of adding longitudinal reinforcement.

$($b$)$ Figure $($b$)$ below is the section of a column with stirrups. Please write down the regulations and reasons for the clear spacing of different sections? $(1\%)$

$($c$)$ The figure $($c$)$ below is the mutual graph of column strength, please write the value $(3\%)$ at the question mark in the figure, and explain under what circumstances it is necessary to modify this curve. $(2\%)$