CPA Problem for Dummies: Understanding Activity Networks

[Nubcake]

This is not really a h/w question but one thing I so not understand with this precedence table in critical path analysis , why would it needs two dummies?Activity. | Depends On
A | -
B | -
C | A
D | B
E | B,C
F | B,C

I drew this activity network and I only used 1 dummy to represent the dependence of E , F didn't need another dummy variable.

Can anyone explain why? Hence draw an activity network to show .

Thanks

 

[Best Pokemon]

You need a dummy starting variable that connects to A and B. You also need a dummy ending variable that is connected from D, E and F.