Definition: In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Input: Question: If an analytical method allows to determine 1 ppm of Pb, we are referring to: 
 Options:  <1> 1 mg / L. <2> 1 mg / mL. <3> 1 μg / L. <4> 1 ng / L. <5> 1 ng / mL.
