This is a problem in which you have to make a comparison about the two spaces given between the points. In the case of the central strip we know that it is of order 2. Therefore, if the strip width is 3 cm, the distance that will exist towards the bright strip of the second order will be the first strip, plus an equidistant 3 cm, that is, a strip of 6cm from the first to the last strip.
The correct answer is B.