Sometimes a research study does not permit an inference that the effect of a treatment differs from zero; these inconclusive studies can be referred to as producing a null result. An "informative null" can be used to refer to a study that had a null result but nonetheless provided information that the treatment effect is at most small. Of the measures below, which measure is most useful for assessing whether a result from a study can be appropriately referred to as an informative null? A.a p-value B.a standard deviation C.a 95% confidence interval