Two long parallel wires carry currents in the same direction. The force between them is:

1. Each wire sits in the magnetic field of the other. 2. Applying $\mathbf{F} = I\mathbf{l}\times\mathbf{B}$ to each wire gives forces directed toward the other wire. 3. Hence parallel (same-direction) currents attract. 4. Antiparallel currents would instead repel, so the zero/perpendicular options are wrong. _Source: syllabus Physics Ch 4 "Moving Charges and Magnetism", p.154_