Final answer:
The Metric System is used to measure length in U.S. scientific investigations, with units such as millimeters, centimeters, meters, and kilometers.
Step-by-step explanation:
In the United States, the Metric System is commonly used to measure length in scientific investigations. While the Customary System is primarily used in the United States for everyday measurements, in scientific fields and internationally, the metric system is preferred due to its ease of conversion and global standardization. The metric units most commonly used to measure length include the millimeter, centimeter, meter, and kilometer. In addition, it is important to know that the Imperial System is similar to the Customary System and was originally used in the United Kingdom, but the Metric System has largely replaced it in scientific contexts.