"D" is the best answer. The actual Vimy planners used all three methods. They analyzed hundreds of air photos of the German defenses, as well as using flash and sound direction and timing and basic geometry to calculate the location of every German gun. Using all three methods allowed the anti-battery gunners to locate German guns day or night. By the time of the actual attack, the Canadian gunners had silenced over 80% of all German guns at Vimy!