Two Parallel Straight Wires Are $7.0 \mathrm {~cm}$ Apart And $50 \mathrm {~m}$

Multiple Choice

Two parallel straight wires are $7.0 \mathrm {~cm}$ apart and $50 \mathrm {~m}$ long. Each one carries a $18 - \mathrm { A }$ current in the same direction. One wire is securely anchored, and the other is attached in the center to a movable cart. If the force needed to move the wire when it is not attached to the cart is negligible, with what magnitude force does the wire pull on the cart? $\left( \mu _ { 0 } = 4 \pi \times 10 ^ { - 7 } \mathrm {~T} \cdot \mathrm { m } / \mathrm { A } \right)$

A) $93 \mathrm { mN }$
B) $66 \mathrm { mN }$
C) $37 \mathrm { mN }$
D) $46 \mathrm { mN }$