23. at the beginning of debussy's prélude à "l'après-midi d'un faune," the flute outlines
the interval of a
octave
unison
tritone
fifth

At the beginning of Debussy's Prélude à "L'Après-midi d'un faune," the flute outlines  the interval of a tritone.

Explanation:

Debussy's Prelude starts with the playing of single flute. There are 3 measures. In the piece of measure that is opening  the interval of tritone is outlined by the flute. This is done by playing scale in that descends  from C-sharp to G and then goes back again to C-sharp.

The fee of an ambiguity is imparted from this interval of tritone. The octave is partitioned into two halves. Here F-sharp, which is a tritone is above one C and a tritone below another C.  This similarity in the interval helps the music listeners to identify the pitch that is very important.  

based on the images pictured above, the major difference between the two cathedrals: the cathedral of notre dame and the amiens cathedral, is that the cathedral of notre dame emphasizes the horizontal lines while the amiens cathedral emphasizes the vertical lines.

trying using google im so sorry