The continuous time unit-step function is defined by

A. \[u\left( t \right) = \left\{ {\begin{array}{*{20}{c}} {1,}&{t > 0} \\ {0,}&{t < 0} \end{array}} \right.\]

B. \[u\left( t \right) = \left\{ {\begin{array}{*{20}{c}} {1,}&{t \geqslant 0} \\ {0,}&{t < 0} \end{array}} \right.\]

C. \[u\left( t \right) = \left\{ {\begin{array}{*{20}{c}} {1,}&{t > 0} \\ {0,}&{t \leqslant 0} \end{array}} \right.\]

D. \[u\left( t \right) = \begin{array}{*{20}{c}} {1,}&{t \ne 0} \end{array}\]

Answer: Option A