The Main Impediment to Fusion

The main impediment to fusion

A) is the fact that these are first-order reactions, so they are kinetically slow.
B) fusion occurs only for a few very rare nuclides.
C) reacting nuclei must have high kinetic energies to penetrate electron clouds.
D) the fusion products are often radioactive.
E) reacting nuclei must have high kinetic energies to overcome electrical repulsion between positive particles.