# Help understanding Biot-Savart Law

In the Biot Savart law, it's my understanding that dL represents an infinitesimally small length of the wire carrying current. See below.

However, I ran into a practice problem that is confusing me. I've pasted it below. In this problem, instead of using length of the wire for dL, they use the diameter of the wire. Why? I thought you use length?

The problem is asking for the "infinitesimal" magnetic field (note they write the final answer as $dB=...$ instead of $B=...$) at the center of this configuration contributed from a 1.22 mm length of wire located at the midpoint of each wire. So then $dl=1.22\space mm$ is the length of this "infinetesimal" wire segment. It is not saying the diameter of the wire is 1.2 mm. The wire is still assumed to be infinitely thin.