What are positive shapes the shapes of artist's canvasesthe main shape in a paintingthe automatic shapes created for backgrounds

Of all these answers, B is the most correct. A positive shape makes a juxtaposition with “negative space”. The main shape is used to bring out detail of its surroundings; to make it “come alive” so to speak.

White flour this don't make sense

Generally spontaneous human combustion is something that still bewilders some, human combustion happens very rarely and doesn't affect the surrounding areas, so unless you want to set your self on fire, human combustion will remain spontaneous and very rare.