Greedy by shortest duration: always pick the shortest activity.
Example: (0,4), (3,5), (4,8). Durations: , , . Greedy picks (3,5) first (duration ). Now (0,4) conflicts (overlaps at 3-4). And (4,8) conflicts (overlaps at 4-5). Greedy gets only activity. But the optimal is (0,4), (4,8) for activities.
Being short does not help if the activity sits in the middle and blocks both sides. Duration alone does not tell you about position. The shortest activity might sit in the worst place.