What is the slack of activity c?

0

4

2

1

\begin{tabular}{|c|c|c|} \hline Activity & Immediate Predecessors & Activity Time (days) \\ \hline A & - & 4 \\ \hline B & - & 7 \\ \hline C & B & 6 \\ \hline D & A, B & 10 \\ \hline \end{tabular}