A Paired Difference Experiment Yielded $\mathrm { n } _ { \mathrm { d } }$

Multiple Choice

A paired difference experiment yielded $\mathrm { n } _ { \mathrm { d } }$ pairs of observations. For the given case, what is the rejection region for testing $\mathrm { H } _ { 0 } : \mu _ { \mathrm { d } } = 15$ against Ha: $\mu _ { \mathrm { d } } < 15$ ?

$\mathrm { n } _ { \mathrm { d } } = 21 , \alpha = 0.05$

A) $t < - 1.721$
B) $t < 2.086$
C) $\mathrm { t } < 1.725$
D) $\mathrm { t } < - 1.725$