QUESTION $2$

Here is a definition for a datatype called Tree which can be used to represent expression trees.

datatype Tree $=$ Leaf of int $|$ Add of Tree $*$ Tree $|$ Sub of Tree $*$ Tree ;

Which of the following represents a complete implementation to evaluate any valid expression tree that takes a Tree and returns an integer?

fun eval$($Leaf$($c$))=$ c

$|$ eval$($Leaf$($c$))=$ eval$($Leaf$($c$))$;

fun eval$($Leaf$($c$))=$ c

$|$ eval$($Leaf$($c$,$d$))=$ eval$($Add$($c$),$Sub$($d$))$;

fun eval$($c$)=$ c

$|$ eval$($c$)=$ eval$($Add$(3,4))$

$|$ eval$($c$)=$ eval$($Sub$(3,4))$;

fun eval$($Leaf$($c$))=$ c

$|$ eval$($Add$($c$,$ d$))=$ eval$($c$)+$ eval$($d$)$

$|$ eval$($Sub$($c$,$ d$))=$ eval$($c$)-$ eval$($d$)$;

fun eval$($Leaf$($c$))=$ c

$|$ eval$($c$,$ d$)=$ c $+$ d

$|$ eval$($c$,$ d$)=$ c $-$ d;