Define RfD and RfC and indicate when each is used.

• A. RfD is a reference dose used for oral chronic exposure (mg/kg-day); RfC is the inhalation counterpart for chronic exposure (mg/m3); both incorporate uncertainty factors.
• B. RfD is for inhalation; RfC is for dermal exposure; no uncertainty factors.
• C. RfD is a measure of acute toxicity; RfC measures surface tension.
• D. RfD and RfC are identical and used interchangeably.

Answer: (A)