According to the explanation given in the passage, which of the following causes superbubb

les to gradually assume the shape of a chimney？

A．The gravitational attraction of supernovas adjacent to the region of the chimney's formation

B．The effect of entropy on the relative abundance of hot interior gas within the superbubble

C．A chemical reaction involving ionized trace ions at the heart of the superbubble

D．A tendency toward coalescence in the gases that contribute to the formation of the superbubble

E．A natural buoyancy of the superbubble, which causes the chimney to stretch and become a gaseous conduit