【单选题】

According to the explanation given in the passage, which of the following causes superbubbles to gradually assume the shape of a chimney
A．(A) The gravitational attraction of supernovas adjacent to the region of the chimney' s formation

B．(B) The effect of entropy on the relative abundance of hot interior gas within the superbubble
C．(C) A chemical reaction involving ionized trace ions at the heart of the superbubble
D．(D) A tendency toward coalescence in the gases that contribute to the formation of the superbubble
E．(E) A natural buoyancy of the superbubble, which causes the chimney to stretch and become a gaseous conduit