The answer is
a). 0λ
b). λ
c). 1/2 λ
The explanation:
-when The speakers represent antinodes of the resulting standing wave pattern.
-when other points along the medium that undergo vibrations between a large positive and large negative displacement. These are the points that undergo the maximum displacement during each vibrational cycle of the standing wave. In a sense, these points are the opposite of nodes, and so they are called antinodes.
-In order to pick up the loudest sound the following distances should work:
a). 0λ
b). λ
c). 1/2 λ