In the structure shown, an $8-$mm$-$diameter pin is used at A$,$ and $12-$mm$-$diameter pins are used at $B$ and $D.$ Knowing that the allowable load $P$ is $80kN.$ Determine the following;

a$)$ The shearing stress in $nA,$

b$)$ The shearing stress in pin $D,$

c$)$ The largest normal stress in lit

d$)$ The bearing stress in the link :