Final answer:
The informal change contributed by increased international interactions is the expansion of presidential powers, attributed to economic, technological, and foreign policy factors.
Step-by-step explanation:
Increased interactions between the United States and other nations have contributed to an informal change in the U.S. system of government, specifically, A. Expansion of the powers of the presidency. This expansion is attributed to several factors, including the complexity of the domestic economy, the increased significance of the U.S. on the international stage, and the technological advancements that have amplified the president's ability to communicate directly with the public. In addition, major events such as wars and economic crises have often led to a boost in presidential stature and a broader interpretation of the executive powers granted in the Constitution. Presidents have used executive orders and executive agreements to act unilaterally, providing interpretations of legislation, formulating budgets, and in cases involving foreign policy and war powers, have sometimes bypassed the traditional treaty process that involves the Senate.