Task: Text Categorization
Input (English): Question: If an analytical method allows to determine 1 ppm of Pb, we are referring to: 
 Options:  <0> 1 mg / L. <1> 1 mg / mL. <2> 1 μg / L. <3> 1 ng / L. <4> 1 ng / mL.
Output (English): chemistry