The trucking industry can be justly called a _______ of the U.S. economy.

The trucking industry is often referred to as the "mainspring" of the U.S. economy because it plays a vital role in the movement of goods and services across the country. Just like a mainspring in a mechanical device, which is crucial for its operation, the trucking industry is essential for the functioning and efficiency of the entire supply chain. It facilitates trade by ensuring that products reach consumers in a timely manner, supporting various sectors such as retail, manufacturing, and agriculture.

When considering the other terms, while "foundation," "backbone," and "support system" also convey the importance of trucking to the economy, "mainspring" specifically emphasizes its dynamic nature and central role in driving economic activity. It highlights how the industry is not just a passive element but an active engine that propels economic growth and development. In this context, the trucking industry is integral to maintaining the rhythm and flow of commerce, akin to how a mainspring energizes the workings of a clock or machinery.