Clue 1: a black line is shorter than a white line
clue 2: the white line is shorter than a gray line.
is the black line or shorter than the gray line?

The black line is shorter than the gray. Let's simplify the rules. (B=Black, W=White, G=Grey

B is less than W which is less than G

That means G is greater than W. B is less than W, which is less than G. Therefore, the black line is not only shorter than the Gray one, but also the shortest out of the three.

I hope this Helps!