Concept:Resonance is the large vibration caused when a periodic force matches the natural frequency of an object.
Explanation:When soldiers march in a regular, orderly rhythm, their steps exert a repeated periodic force on the bridge.
If this rhythm matches the natural frequency of the bridge, resonance occurs.
The vibrations grow larger with each step, and the bridge may eventually collapse.
Marching disorderly breaks this regular rhythm, so no sustained resonance is set up.
This protects the bridge from dangerous oscillations.
Therefore, the main reason is to prevent resonance on the bridge.
Answer:A. To prevent resonance on the bridge.