$(15$ points$)$ Consider the poset $),$ where $M=\{2,4,6,8,10\},N=P(M)-M,.$

$($a$)$ Write down all the maximal element$($s$).($Write down 'Nil' if there isn't any maximal element.$)$

$($b$)$ Write down all the minimal element$($s$).($Write down 'Nil' if there isn't any minimal element.$)$

$($c$)$ Write down all the upper bound$($s$)$ of $\{\{6\},\{2,8\}\}.($Write down 'Nil' if there isn't any upper bound.$)$

$($d$)$ Write down all the lower bound$($s$)$ of $\{\{2,6,8\},\{4,6,8\},\{2,6,8,10\}\}.($Write down 'Nil' if there isn't any lower bound.$)$