$(15$ points$)$ Consider the poset $,|).$

$($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 $\{12,18\}.($Write down 'Nil' if there isn't any upper bound.$)$

$($d$)$ Write down all the lower bound$($s$)$ of $\{48,60\}.($Write down 'Nil' if there isn't any lower bound.$)$