The correct answer is $\mathrm{B}$. This question relies on a knowledge of the basic chemical composition of the biologically important molecules. Carbohydrates and lipids are composed of C, O, and H. However, DNA is a nucleic acid and contains phosphate groups.
A scientist suspects that the food in an ecosystem may have been contaminated with radioactive phosphates over a period of months. Which of the following substances could be examined for radioactive phosphate to test the hypothesis?
A. the carbohydrates produced by plants in the area
B. the DNA of the organisms in the area
C. the lipids produced by organisms living in the area
D. all of the above