A technical spider laid out her large plane web on a Cartesian co-ordinate system, as shown below. She located two identical small speakers, one at (X=-3m,Y=8m) and the other at (X=3m, Y=-8m).
She drove both speakers with the same electrical signal, so they sent out identical sound waves of frequency 85Hz, attractive to flies.
A bold fly walked slowly along the X axis from X=-100m to X=100m, listening to the resulting sound.
How many local maxima did the fly detect? Use 340m/s for the speed of air in sound if you need it.
(a) 1
(b) 2
(c) 3
(d) 4
(e) 5
