What, according to the passage, has changed in sousaphone design over time?

Bell diameter and piston standardization-(a) and (b) make fair assumptions, but neither is supported by the text. likewise, the passage makes no mention of a change in bell position with respect to the players head, ruling out (d) as a good choice. (e), like (a) and (b), makes a reasonable inference, that if the sousaphones bell changed its sound diffusion properties ought to change, too; however, the question asks about a change in design, not a change in instrument performance(e) answers the wrong questions. this leaves (c), which correctly identifies two aspects of design that the passage explicitly states changed over time.