Microeconomy questions please help

Question

1) Why is marginal cost also opportunity cost?

2) Why do you suppose that advocates of a decision often ignore all the consequences?

3) How does economic analysis trace the effects from a decision?

Answer 1

1) Each extra/additional cost/investment/expenditure has potential alternative usage.

2) Because they have interest only in getting benefit from decision, and may not necessarily bear consequences by themselves (it is very popular in politics, but in economy/management this pattern is common too, for instance for temporary/short-run managers, etc.).

3) Before outcome - using estimates, probabilities and models, but after event/effect gathers information/feedbacks and corrects model accordingly.