In an Activity-on-Arrow network, a dummy activity is an artificial, dotted-line connector that consumes no time—used solely to preserve logic and uniqueness of event numbering.

Introduction / Context:
Dummy activities are unique to certain network notations (especially AOA). They do not represent real work but ensure correct logical relationships and avoid ambiguity in event numbering.

Given Data / Assumptions:

• Notation: Activity-on-Arrow (AOA) with events as nodes, activities as arrows.
• Need to represent logic like common starts/finishes without implying work.

Concept / Approach:
A dummy is inserted to maintain precedence (e.g., to distinguish parallel activities or enforce that one activity must finish before another starts) without adding duration or resources. Conventionally, dummies are drawn as dotted arrows and carry zero duration.

Step-by-Step Solution:
1) Identify a logic need that cannot be represented by existing real activities.2) Insert a dummy arrow to encode that logic (e.g., finish-to-start tie).3) Ensure dummy duration = 0 and representation = dotted line.4) Recalculate times; dummies do not change durations but affect paths and floats.

Verification / Alternative check:
Check that after inserting the dummy, each activity has unique start–finish events and the intended precedence is unambiguous.

Why Other Options Are Wrong:

• Options A–C are all true; picking one would omit essential aspects.

Common Pitfalls:

• Assigning nonzero time to dummies, corrupting path calculations.
• Overusing dummies instead of restructuring network logically.

Final Answer:
all the above.