Task: Question Answering
Input (English): Question: In the respiratory chain: 
 Options:  <1> Ferrosulfurized proteins contain heme groups. <2> The heme groups are in carotenoids. <3> Cytochromes contain iron and sulfur groups. <4> Ubiquinones transport protons and electrons. <5> All transporters carry two electrons.
Output (English): 4