Making rational decisions "at the margin" means that people
a. compare the marginal costs and marginal benefits of each decision.
b. evaluate how easily a decision can be reversed if problems arise.
c. make those decisions that do not impose a marginal cost.
d. always calculate the dollar costs for each decision.

a. Compare the marginal costs and marginal benefits of each decision.

Explanation:

First of all, thinking at the margin is the explanation of how you think about your next step forward. Generating from margin word, marginal means additional. You utilized thing and then utilize additionally but not the same as before, just the residual thing you need. After all, marginal cost, revenue and etc. originated from that. While you are making decisions and fortunately you are rational so you will do systematically and purposefully the best they can to achieve their objectives, given the available opportunities. Subsequently, you will take an action if and only if the marginal benefit of the action exceeds the marginal cost. Because you need estimate what the cost which is marginal one will wait for you and what about your benefit that also will come to you marginally.

answer; by looking at total product,marginal product,and changes in both after changes in inputs

answer; no individual buyer or seller has any significant impact on the market price;