$*$ If a $400-$foot$-$long vertical curve joins an initial grade line of $+3\%$ with a second grade line of $+5\%,$ what is the percent change in grade per station?$*$ A$.0\mathrm{.}50\%*$ B$.0\mathrm{.}75\%*$ C$.1\mathrm{.}25\%*$ D$.2\mathrm{.}0\%$