An acceptable activity-on-arc construction is shown in the diagram.
Official marking guidance for 4(a), 5 marks:
M1: Nine activities labelled on arcs, one start and at least two dummies placed. Activity on node is M0.
A1: A, B, the first dummy with its correct arrow, C, D, E and H dealt with correctly.
A1: The second dummy with its correct arrow, F, G, I and J dealt with correctly.
A1: K, L and M dealt with correctly; the third and fourth dummies with arrows are required.
A1: Correct solution only: all activity arrows correctly placed, one finish and at most four dummies.
Dealt with correctly means that the activity starts from the correct event but need not necessarily finish at the correct event. Incorrect or missing event numbers are condoned.
If E is unlabelled but its arc is present, the first and final A marks are lost; the second A mark may still be earned on benefit of doubt. With two or more unlabelled arcs, mark according to the scheme. A solid unlabelled line is an activity, not a dummy.
Incorrect or missing activity arrows are ignored for the first four marks only. If there are no arrows on any dummies, award M1 only. Additional unnecessary dummies that preserve precedence lose only the final A mark, if earned.