113. Read the following passage, then answer the question below.

In the early 1990s, the anthropologist Robin Dunbar noticed that, across primate species, the typical size of a social group rises with the size of the neocortex, the part of the brain involved in complex thinking. Applying the pattern to humans, he predicted that a person can sustain stable relationships with about 150 individuals. The figure, now called Dunbar's number, seemed to turn up in places he had not originally looked, including the size of some traditional villages and of the personal networks that people report in surveys.

Dunbar offered an explanation tied to time. Monkeys and apes maintain alliances by grooming one another, a slow activity that limits how many partners an animal can keep. Humans, he suggested, replaced grooming with language, since conversation and gossip can be shared with several people at once. This freed time and allowed groups to grow, although not without limit, because remembering who is friendly with whom becomes harder as the web of relationships widens. He also observed that relationships are layered: a few intimate friends, a somewhat larger circle of good friends, and so on outward, with each layer roughly three times the size of the one inside it.

The number has been criticized. Some researchers who reanalyzed the primate data argued that the statistical uncertainty is so large that the true value could lie anywhere from a handful to several hundred, which makes a single figure misleading. Others note that cultures differ in how people define a friend. Even so, the idea has proved influential in the design of organizations, where managers sometimes split large teams into units small enough for members to know one another. Whether the number is a fact about human biology or merely a convenient rule of thumb remains unsettled.

The phrase 'remains unsettled' in the last sentence is closest in meaning to