When an army crosses a bridge, soldiers are instructed not to march in step to avoid the danger of the bridge collapsing due to resonance; option [B]. Resonance occurs when the frequency of the soldiers’ synchronized steps matches the bridge’s natural frequency, causing the vibrations to amplify significantly. This can lead to large oscillations and potentially catastrophic structural failure of the bridge. Marching in step, especially if the bridge is long or has a specific structural design, can inadvertently set up these dangerous oscillations. This precaution is crucial because historical incidents have demonstrated that even well-built bridges can succumb to resonant vibrations under synchronized forces. By breaking step, the soldiers create irregular, non-periodic forces on the bridge, which minimizes the risk of resonance and ensures the structural integrity of the bridge remains intact during the crossing.