Question 2
Nucleic acids contain C, H, O and N — and one more element the other biological molecules do not have. Which?
The right answer is the one marked above.
Phosphorus (P). It is the element that marks out a nucleic acid.
Use the elements as a decision tree. C, H, O only → carbohydrate or lipid. Add nitrogen → protein. Add phosphorus as well → nucleic acid. This is also why the mineral salt table calls phosphorus "an essential component of nucleic acid".