Consider the hydrolysis of geranyl pyrophosphate to geraniol shown in Eq$.17\mathrm{.}46$ on p$.863.$

$($a$)$ What isotopically substituted derivative of geranyl pyrophosphate could be used to establish the assertion that this hydrolysis proceeds by opposite$-$side substitution?

$($b$)$ Given that the isotopically substituted derivative you proposed in $($a$)$ can be prepared in enantiomerically pure form with a known absolute configuration, the absolute stereochemistry of the product geraniol must be determined to establish the stereochemistry of substitution. Explain how the alcohol dehydrogenasecatalyzed oxidation of the isotopically substituted geraniol product by $NA{D}^{+}($Sec$.10\mathrm{.}9$B$)$ can be used to establish its stereochemistry.