Final answer:
The proportion of senate terms that expires every two years is one-third.
Step-by-step explanation:
The proportion of senate terms that expires every two years is one-third. This means that only one-third of the Senate is up for election every two years. In the United States Senate, there are 100 senators, with two senators from each state. Each senator's term lasts six years, and the terms are staggered so that one-third of the Senate is up for election every two years.