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: $\mu _ { \mathrm { d } } < 15$ ?
$$\mathrm { n } _ { \mathrm { d } } = 19 , \alpha = 0.01$$

A) $t < 2.552$
B) $t < - 2.552$
C) $t < 2.861$
D) $t < - 2.539$