Task: Question Answering
Input (English): Question: The thermal conductivity detector was one of the first used in gas chromatography and still has a lot of application. Consists in: 
 Options:  <1> A source that is heated by electricity, whose temperature at a constant electrical energy depends on the thermal conductivity of the gas that surrounds it. <2> A metallic surface on which ions collide, producing emission of electrons or other ions, with the consequent variation in conductivity. <3> An air / hydrogen flame where organic compounds are pyrolyzed producing ions that lead to a change in conductivity. <4> An emitter of beta radiation that allows the measurement of the conductivity of the eluent that leaves the chromatographic column. <5> A source of thermal ionization that produces positive and negative ions that change the conductivity of the gas passing through the detector.
Output (English): 1