Gelatins of three densities are poured into a large pan. Gelatin I is a diluted and soft gelatin, Gelatin Il is a regular, firm gelatin, and Gelatin III is an extra firm, rubbery gelatin. A pulse in the shape of a crest is generated in III. A pulse (wavelength) is generated that extends exactly 1 color-band, that is, over 1 section of gelatin. In other words, the first
pulse generated extends from the back end of the pan to the red/green boundary.

Choose whether the pulse will transmit as a trough or a crest under the following conditions.

At boundary A, the transmitted pulse will be a