Task: Question Answering
Input (English): Question: The method of continuous variations is used in molecular absorption spectrophotometry to: 
 Options:  <1> Determine the position of the isobestic point in a mixture of absorbent species. <2> Calculate the exact position of the absorption maximum of an absorbent complex. <3> Obtain the pH corresponding to the end point of the titration of an absorbing acid or base. <4> Calculate the stoichiometry of an absorbent complex. <5> Calculate the exact value of the dissociation constant of an absorbent acid.
Output (English): 4