$18\mathrm{.}8$

For each of the following pairs of compounds, indicate

whether the pair represents different compounds that are

$(1)$ constitutional isomers or $(2)$ stereoisomers that are

enantiomers or $(3)$ stereoisomers that are diastercomers or

$(4)$ not isomers. Refer to Figures $18\mathrm{.}1$ through $18\mathrm{.}4$ as needed

for the structures of the compounds. $($a$)$ D$-$Glucose and

$3-$deoxy$-$D$-$glucose; $($b$)$ D$-$ribose and D$-$glucose; $($c$)$ D$-$mannose

and L$-$mannose; $($d$)$ D$-$galactose and $-$mannose; $($e$)$ D$-$galactose

and D$-$fructose.