What deployment flaw could cause this problem?

Two co-located 802.11b/g/n APs can interfere with one another and cause contention and collisions, even when the two APs are operating on non-overlapping channels (e.g. 1 & 6).

What deployment flaw could cause this problem?
A . The access points are mounted too closely to one another.
B . Reflective objects in the area are causing significant multipath.
C . A client station is using active scanning to probe for access points on multiple channels.
D . The access points are not synchronized to the same NTP server.

Answer: A