Chemical Manufacturers Association (CMA) and four companies that manufacture chemicals challenged a test rule promulgated by the Environmental Protection Agency (EPA) under the Toxic Substances Control Act (TSCA). The plaintiffs asserted that the EPA must find that the existence of an unreasonable risk of injury to health is more probable than not before it may issue a test rule under the Act. In response, the EPA claimed that it may issue a test rule under the TSCA if the agency determines that there is a substantial probability of an unreasonable risk of injury to health. The test rule required toxicological testing to determine the health effects of the chemical, 2-ethylhexanoic acid, and imposed on exporters of this chemical a duty to file certain notices with the EPA. What standard should be applied? Why?