Scientist: In our study, chemical R did not cause cancer in laboratory rats. But we cannot conclude from this that chemical R is safe for humans. After all, many substances known to be carcinogenic to humans cause no cancer in rats; this is probably because some carcinogens cause cancer only via long-term exposure and rats are short lived.
It is cited as being insufficient to support the conclusion that chemical R is safe for humans most precisely describes the role played in the scientist's argument by the statement that chemical R did not cause cancer in laboratory rats.