Toxic substances, toxicants, or poisons are defined as what?

Toxic substances, toxicants, or poisons are defined as what?
 
  a. Substances that cause death, illness, injury, or incapacitation when they are exposed to the body in relatively small quantities
  b. Ingested substances that cause death, illness, injury, or incapacitation when they are exposed to the body in relatively small quantities
  c. Inhaled substances that cause death, illness, injury, or incapacitation when they are exposed to the body in relatively small quantities
  d. Injected substances that cause death, illness, injury, or incapacitation when they are exposed to the body in relatively small quantities