求教一道填空题，谢谢！

ysm12345

请问第二空为什么选E呢？

3.      At one time researchers viewed the placebo effect as (i)_____,a statistical (ii)_____ faced by those attempting an objective evaluation ofthe efficacy of potentially legitimate therapies. That view has changed: the placebo effect istoday seen as an important part of the healing process.

A. a boon	D. increment
B. a touchstone	E. annoyance
C. an impediment	F. deviation