Gelatins of three densities are poured into a large pan. Gelatin I is a diluted and soft gelatin, Gelatin II is a regular, firm gelatin, and Gelatin III is an extra firm, rubbery gelatin. A pulse in the shape of a trough is generated in 1. 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 front end of the pan to the blue/red 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
.