Based on the scenarios provided, the most directly affected spheres immediately following each incident would be:
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Train Derailment with Chemical Spill: This scenario would primarily involve the hydrosphere due to the chemicals spilling onto the highway and potentially entering nearby water systems during the rainstorm. It also affects the geosphere (the ground) where the chemicals have spilled. However, the immediate impact is most significant in the hydrosphere. So, the combination would be geosphere and hydrosphere.
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Ammonium Nitrate Explosion: An explosion at the facility would directly affect the atmosphere through debris and smoke being thrown into the air. The incident may also affect the geosphere (ground) due to potential contamination from debris. Thus, the impactful response here could be geosphere and atmosphere.
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Acid Snow and Spring Melting: Acid snow affects the hydrosphere as the melted snow flows into the local river. It also influences the geosphere where the snow falls and can impact the biosphere (local ecosystems and wildlife) through the acidic runoff. Therefore, a possible response could include biosphere and hydrosphere.
Given the goal is to find the combination of spheres most DIRECTLY affected immediately following the incident, the final combination for the train derailment scenario is geosphere, hydrosphere, atmosphere, biosphere or particularly emphasizing: geosphere and atmosphere.