$($a$)$ The chromatogram below shows a Reversed$-$Phase Liquid chromatography $($RPLC$)$

gradient to separate a mixture over $40$ minutes. The mobile phase consisted of

$H_2\frac{O}{?}$ Acetonitrile $(5-95\%).$

All peaks eluted between $17\mathrm{.}2$min and $34\mathrm{.}7$min.

$($i$)$ Using the chromatographic details given above, determine if isocratic or gradient

elution is more suitable for the separation of this mixture.