Hmm.. I was trolling, thought that C would be the worst answer. I would think increasing diameter and decreasing the length of the tube all the way from basic acoustics principles (so probably A from the choices). I wouldn't think that the canal length rationale is applicable as I was under the impression that proximity to the TM contributes in frequency response (ie, prob mic measures with insufficient depth into canal).. I should probably go ahead and schedule my remediation date for comps eh?