Final answer:
In the northern U.S., annual bluegrass is a cool-season grass, thriving during the cooler parts of the year within temperate grassland regions such as prairies. While perennial behavior is suggested, there is not enough information to conclude this definitively. The correct option is b) Cool-season grass
Step-by-step explanation:
In the northern U.S., annual bluegrass exists as a cool-season grass. Cool-season grasses grow primarily during the cooler parts of the year, such as spring and fall, making the best use of cool temperatures and moisture levels during those times. This type of grass is in contrast with warm-season grasses, which grow predominantly in the summer, and biennials, which complete their lifecycle in two seasons.
The northern U.S. falls within the temperate grasslands, which include prairies in North America. In these regions, plant growth aligns with the warmer seasonal temperatures present in spring, summer, and early fall, matching the growth pattern of cool-season grass.
Perennial grasses, by definition, live for multiple years, and the question does not provide enough information to determine if annual bluegrass in the northern U.S. acts as a perennial or not. However, the given excerpt about the grass flowering earlier with each passing year suggests a potential perennial behavior, but again, this is not definitively confirmed. The correct option is b) Cool-season grass