Which action can the engineer take to most accurately calibrate the size of the floor plan in Air Magnet?

An engineer receives a digital image scanned from the floor plans of a facility to be surveyed for wireless survey and imported it into Air Magnet Pro.

However, the document contains no scale.

Which action can the engineer take to most accurately calibrate the size of the floor plan in Air Magnet?
A . Mark the length of a hallway, then count the ceiling tiles, multiply that number by 2 and enter that value.
B . Zoom in and mark across a hallway, then count the floor tiles across that hallway and enter that value.
C . Zoom in and mark a doorway, then size it at 3 feet because most doorways are 36 inches.
D . Mark the entire longest dimension of the floor plan, then use Google Earth to measure the corresponding outside dimension and enter that value.

Answer: D