Ten kilometers __________ a long distance to walk.

The correct answer is C: is. This question tests subject-verb agreement, specifically when a plural noun refers to a single unit of measurement, distance, time, or money. Even though "kilometers" is plural, "Ten kilometers" in this sentence is treated as a single, collective unit representing a specific distance. When such a quantity is considered as one entity or amount, the verb should be singular. Think of it as "This distance (ten kilometers) is long." Other examples include "Five dollars is not enough," "Two hours is a long time to wait," or "Fifty pounds is a heavy weight." The key is to identify whether the plural subject is functioning as a single concept or a collection of individual items. In this case, "a long distance" clearly indicates that "Ten kilometers" is being viewed as one unified measure, thus requiring the singular verb "is."