Consider the following statements regarding updating of the network:
1. For short duration project, updating is done frequently
2. For large duration project, frequency of updating is decreased as the project is nearing completion
3. Updating is caused by overestimated or underestimated times of activities
4. The outbreak of natural calamity necessitates updating
Which of the above statements are correct?
(a) 1, 2 and 3 (b) 2, 3 and 4
(c) 1, 3 and 4 (d) 1, 2 and 4