smc

Let $r$ be the straight road, $G$ be the girls' camp, $B$ be the boys' camp, and $C$ be the water canteen. $\overline{AG}$ is the perpendicular from $G$ to $r$. Suppose each rod is one unit long. $AG=300$ and $BG=500$. Since $\angle GAB$ is a right angle, $△GAB$ is a $3-4-5$ right triangle, so $AB=400$. Let $x$ be the distance from the canteen to the girls' and boys' camps. We have $BC=CG=x$ and $AC=400-x$. Using the Pythagorean Theorem on $△GAC$, we have $$300^2+(400-x{)}^2=x^2$$ Simplifying the left side gives $$90000+160000-800x+x^2=x^2$$ Subtracting $x^2$ from both sides and rearranging gives $$800x=250000$$ Therefore, $x=\frac{250000}{800}=312.5$. The answer is $\boxed{\text{none of these}}$.