1. The problem statement, all variables and given/known data 2. Relevant equations Biot-Savart's law and ampere's law expressing B = mu0 * I / (2 * pi * r) 3. The attempt at a solution I know that for the first drawing, we have magnetic fields going into the page from both of the wires, and we have them being (infinitely) long, so we just sum the two expression from ampere's law to get B0 at the origin = 2 * mu0 * I / (2 * pi * r ) In the right-hand drawing, we have half of each wire placed on the x-axis, and the rest goes parallel to the y-axis vertically. I know that by biot-savart's law, which has a cross product between dl and the r-roof vector, the magnetic field contributed by wires on the x-axis is zero as the equation becomes zero for each part. OK. I know that the wires on the x-axis do not contribute to a magnetic field. I examine the vertical wires going up and down in the second drawing. They both contribute a magnetic field as can be seen by examining biot-savart. However, I am confused... why is the right answer C? Is it because that only half of the length wires contributes a magnetic field compared due to half of them running vertically due to them being bent? Or is there another reason? Many thanks in advance for your help, and have a good day.