Answer
$T\gt T_\frac{α}{2}$: null hypothesis is not rejected.
There is not enough evidence to conclude that $M_D\ne0$.
Work Step by Step
Two-tailed test.
$T_+\lt|T_-|$. So:
Test statistic: $T=T_+=50$
Critical value: $T_\frac{α}{2}=40$
(According to table XII, for $n=18$ and $\frac{α}{2}=0.025$)
Since $T\gt T_\frac{α}{2}$, we do not reject the null hypothesis.
