Which of the Following Statements Is True About Determining Exposure

Which of the following statements is true about determining exposure levels of toxins? 

A)  Since there is considerable uncertainty involved, exposure levels are set at 1/100 or even 1/1000 of the level expected to cause harm. 
B)  Since toxic exposure levels can be determined with great accuracy, limits tend to be set just under the harmful level to accommodate the chemical and pharmaceutical industries. 
C)  Since the actions of toxins, and combinations of various toxins, are well understood, there is no need for continued toxicity studies involving humans or other animals. 
D)  Citizens can rest assured that there will be no surprises arising from the toxic effects of chemicals, or combinations of chemicals, because of the extreme certainties arising from the toxicity studies done to date.