Why is it easier to tell the difference between an ellipsoid height and an elevation in meters near the coast of Maine vs$.$ western Maine?

$($a$)$ geoid height always becomes further from zero near sea level

$($b$)$ elevation differences are more pronounced in western Maine

$($c$)$ more gravity measurements are off$-$shore than on$-$shore in Maine

$($d$)$ ellipsoid heights are negative near the coast in Maine

A GNSS gravity$-$based approach to elevation determination in the future theoretically eliminates the need for

$($a$)$ a geoid model

$($b$)$ CORS stations

$($c$)$ monumented benchmarks

$($d$)$ least squares adjustment of GPS vector networks

Select the best indicator of a quality vector solution

$($a$)$ PDOP

$($b$)$ fixed ambiguity solution

$($c$)$ number of satellites

$($d$)$ multipath

An OPUS$-$S solution produced ellipsoid heights from the $3$ CORS stations used of $\backslash (13\mathrm{.}134,13\mathrm{.}162\backslash ),$ and $13\mathrm{.}140$ meters respectively. The error estimate in the OPUS report next to the ellipsoid height would be closest to

$($a$)\mathrm{.}03$ m $.$

$($b$)\mathrm{.}02$ m $.$

$($c$)\mathrm{.}01$ m $.$

$($d$)\mathrm{.}00$ m $.$