LD 50 of 25 mg/m3 means that a concentration of 25 mg/m3 of a certain substance indicates:

a. 50 people out of a group of 80 exposed to a concentration 25 mg/m3 of a substance were shown to have experienced a fatal dose.
b. 50% of the concentration of a substance cause fatality in those be exposed.
c. A concentration of 25 mg/m3 of a substance is shown to have been fatal to 50 percent of a test group of animals.
d. LD 50 is a fictitious term and does not indicate anything.