Key idea: This is a procedural decision question based on the Final Checklist. It tests optimization and workflow order, not just the final answer.
Step 1: Analyze the motion type.
Trains move in the SAME direction.
Checklist Rule: Relative Speed = Difference of speeds.
This eliminates Option A which adds speeds.
Step 2: Analyze unit requirements.
Lengths are in meters. Time is requested in seconds. Speeds are in km/h.
Conversion is mandatory.
Step 3: Evaluate conversion efficiency (The Checklist Layer).
Method 1 (Option B): Convert 90β25 m/s. Convert 72β20 m/s. Subtract: 25β20=5 m/s. Two conversions, one subtraction.
Method 2 (Option D): Subtract in km/h first: 90β72=18 km/h. Convert once: 18Γ185β=5 m/s. One subtraction, one conversion.
Step 4: Apply Checklist Priority.
The checklist prioritizes minimizing operations to reduce error risk. Converting the difference is computationally superior and less prone to fraction errors than converting two separate larger numbers.
Therefore, Option D is the strict next best step per optimized protocol.
Step 5: Reject distractors.
Option C uses average speed, which is never valid for crossing time.
Answer: Option D.