Choose the One Alternative That Best Completes the Statement or Answers

Choose the one alternative that best completes the statement or answers the question.
-The flat line at the left in (b) probably represents ________________.

A) the fact that there is no biological effect of the toxicant at low doses
B) that no data was collected for the flat line area
C) the bodyʹs immune system and other defenses which are deployed against the toxicant
D) competing effects of some other substance whose effect wasnʹt measured
E) the fact that no toxicant was introduced until the response line begins to rise