How Can One Determine the Level of Influence an Interest

How can one determine the level of influence an interest group has had in foreign policy decision making?

A) One can assume that,since a policy decision benefited a group,the decision must have been made in order to benefit that group.
B) In democracies,interest groups have to make their campaign contributions public,so one can link the amount of money with policy decisions that benefit a group.
C) Measuring the influence of an interest group is difficult,because usually one cannot directly observe the connection between a group and the ultimate decision.
D) Scholars find out how much influence an interest group has had when official records are made public long after the decision has been made.