In russian churches the number of domes usually matches the number of altars. but this church has only one altar dome, and 21 symbolic ones. what normal construction device does this church lack?

the answer is b. i'm positive it's that answer.

including the octave, the answer would 8 notes in a major scale with one note being played twice. ex) c major scale includes: c, d, e, f, g, a, b, c

notice that c is played twice

however, if you were to ask me how many different notes are in a major scale- the answer would be 7.  

because of the importance of consistency in data collection. but we have to evaluate the resource or information, and not just accept what is posted online since information available on the internet is not regulated for quality or accuracy.