Task: Question Answering
Input (English): Question: Of the following reaction mechanisms, in which there is an inversion of the configuration from the reactant to the product of the reaction ?: 
 Options:  <1> Bimolecular nucleophilic substitution (SN2). <2> Unimolecular elimination (E1). <3> Unimolecular nucleophilic substitution (SN1). <4> Addition Markovnikov. <5> Addition anti-Markovnikov.
Output (English): 1