Fact 1: Cells secrete acids and bases to maintain the proper pH for enzymes to work.

Fact 2: An enzyme is a biological catalyst .

Given the two facts above, answer the question "Catalysts require proper what to work?" with the following options: 
- DNA
 - pH
 - pan
 - RNA
 - elevation
 - time of day
 - gas
 - humidity


 

  pH       