\(d:\left\{ \begin{array}{l}x = {x_0} + at\\y = {y_0} + bt\\z = {z_0} + ct\end{array} \right.\left( {t \in \mathbb{Z}} \right)\)            

\(d:\left\{ \begin{array}{l}x = {x_0} + at\\y = {y_0} + bt\\z = {z_0} + ct\end{array} \right.\left( {t \in \mathbb{R}} \right)\)

\(d:\left\{ \begin{array}{l}x = a + {x_0}t\\y = b + {y_0}t\\z = c + {z_0}t\end{array} \right.\left( {t \in \mathbb{R}} \right)\)            

\(d:\left\{ \begin{array}{l}x = a + {x_0}t\\y = b + {y_0}t\\z = c + {z_0}t\end{array} \right.\left( {t \in \mathbb{Z}} \right)\)