The figure below has been reproduced from a $2007$ study conducted by researchers from University of Schleswig$-$Holstein $($Germany$)$ and shows the resting membrane potential $($RMP$)$ of the atrium $($A$)$ and ventricle $($ V $)$ of the heart of mice during early $($ED$)$ and late $($LD$)$ development. As you can see, there was a significant difference in RMP between ED and LD in both the atrium and ventricle, which the researchers showed could be entirely explained by differences in the activity of the sodium$-$calcium exchanger $($NCX$).$ Given that NCX exchanges three Na$+$ ions for every one Ca$2+$ ion, in which direction must the NCX operate during development in mouse hearts?

$$ forward $($i$.$e$.,$ Na$+$ in$,$ Ca$2+$ out$)$

$$ reverse $($i$.$e$.,Na+$ out, Ca$2+$ in$)$