Determine whether the given vectors are orthogonal, parallel, or neither.
$$
\begin{array}{l}{\text { (a) } \mathbf{u}=\langle- 5,4,-2\rangle, \quad \mathbf{v}=\langle 3,4,-1\rangle} \\ {\text { (b) } \mathbf{u}= 9 \mathbf{i}-6 \mathbf{j}+3 \mathbf{k}, \quad \mathbf{v}=-6 \mathbf{i}+4 \mathbf{j}-2 \mathbf{k}} \\ {\text { (c) } \mathbf{u}=\langle c, c, c\rangle, \quad \mathbf{v}=\langle c, 0,-c\rangle}\end{array}
$$