Track / Stage 4 / 66 of 340 #326 of 2000

4. If $m>0$, and the slope of the line passing through the points $(m, 3)$ and $(1, m)$ is $m$, then $m=($ ).
(A) 1.
(B) $\sqrt{2}$.
(C)
(D)?.
(E) $\sqrt{5}=$

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

I solved it I didn't Skip

Official solution

$\frac{4}{C}$

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.