If a gravity determination was made at an elevation of $250$ m in Grand Forks ND $($Latitude of $\backslash (47\mathrm{.}91957^\{\backslash$circ$\}\backslash$mathrm$\{$N$\}\backslash ),$ Longitude $\backslash (97\mathrm{.}06821^\{\backslash$circ$\}\backslash$mathrm$\{$W$\}\backslash )),$ what is the value of the free$-$air correction?

NOTE: Use the full FA correction equation; notice that you get a negative number for a positive elevation, however for positive elevations $($above sea level$)$ we have to use the positive of that FA corr $\backslash (^\{2\}\backslash )$ value that you find... that is why the "super simple" FA correction gives positive numbers for positive elevations.

Approximately $77\mathrm{.}2$ mGals

Approximately $87\mathrm{.}4$ mGals

Approximately $57\mathrm{.}2$ mGals

Approximately $67\mathrm{.}6$ mGals