Given the following small project for an AOA network:

$\backslash$table$[[$Activity$,$Immediate Predecessor$($s$),$Time Duration $($days$)],[$A$,$D$,7],[$B$,$D$,6],[$C$,$A$,$ B$,5],[$D$,$None,$4],[$E$,$D$,13]]$

How many dummy arc$($s$)$ in this AOA network should be there totally?

Select one:

a$.$ two dummy arcs

b$.$ no dummy arc

c$.$ four dummy arcs

d$.$ one dummy arc

e$.$ three dummy arcs