Task: Question Answering
Input (English): Question: In suppression ion chromatography: 
 Options:  <1> A single column is used and the conductivity of the mobile phase is kept very low. <2> The eluent must be able to be removed selectively after separation and prior to the conductometric measurement. <3> The eluent incorporates a regenerating solution of strong acid flowing in the same direction as the mobile phase. <4> Weak exchangers are used from polystyrene polymers. <5> A mobile phase with ionic conductivity gradient is used.
Output (English): 2