= Solution
The range of a force is controlled by the lightest state that can carry it. Exchange of a particle of mass $m$ produces a <Yukawa potential> proportional to $e^{-mr}/r$, with range $\hbar/(mc)$. The <electromagnetic interaction> is long range because its carrier, the <photon>, is massless. The <weak interaction> is short range because the <W bosons> and <Z boson> are massive. Although the <gluons> of the <strong interaction> are massless in the Lagrangian, <Quantum chromodynamics> confines color and has a <mass gap>; only color-singlet hadrons propagate over macroscopic distances. The residual nuclear force is consequently controlled at long distance by massive pion exchange.
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