The LD 50 is a number scientists use in research to determine the dose of a toxin required to kill 50% of a population of organisms. Biologists in Louisiana have become concerned about cadmium levels in the water near New Orleans, and the chance that frogs will die out.

Scientists have performed an experiment on bullfrog tadpoles In Louisiana and the results are presented in the graph. The LD 50 for bullfrogs is reached

A) at about 6 mg per gallon per day.
B) at about 60% of surviving tadpoles.
C) somewhere around 11 mg per gallon per day.
D) somewhere between 2 mg and 3 mg per gallon per d