What does this indicate about the signal strength?

After the completion of a site survey with Ekahau Site Survey tool, using the default color palette, it is noted that multiple areas are shown as white on the heat map when viewing 5 GHz signal strength data.

What does this indicate about the signal strength?
A . The area is below the minimum threshold configured on the tool.
B . The area is below the detectable level and indicates no RF signal.
C . The area is below -100 dBm at coverage cell edge.
D . The area is below -67 dBm at coverage cell edge.

Answer: B