Two identical loudspeakers separated by distance d emit 200Hz sound waves along the x-axis. As you walk along the axis, away from the speakers, you don't hear anything even though both speakers are on. What are the three lowest possible values for d? Assume a sound speed of 340m/s.

So three lowest possible values of d ar 0.85m, 2.55m and 4.25m

Explanation:

The wavelength of wave:

λ=v/f

λ=340/200=1.7m

The destructive interference condition is:

Δd=(m+1/2)λ             where m=0,1,2,3

For minimum destructive interference ,the value of m is equal to zero

Δd=(0+1/2)×1.7

Δd=0.85m

For m=1

Δd=(1+1/2)×1.7

Δd=2.55m

For m=2

Δd=(2+1/2)×1.7

Δd=4.25m

So three lowest possible values of d ar 0.85m, 2.55m and 4.25m

your answer is c! hope this : )