Which of the following does not describe acute exposure to hazardous materials?

a. determination of exposure is almost always after the fact.
b. long time period between exposure to an agent and the onset of symptoms.
c. short term period between exposure and the onset of symptoms
d. usually a one time or few times event that is also usually unexpected

The correct option is B.
Acute exposure refers to the exposure to toxic effects of hazardous materials in which there is a short duration of time between exposure and the onset of symptoms. Chronic exposure on the other hand refers to long term effects of the hazardous materials and they are usually of long duration. The option given in  B therefore describe chronic exposure not acute exposure.