Task: Question Answering
Input (English): Question: If the arterial pO2 decreases to 60 mm of Hg: 
 Options:  <1> Decreases ventilation. <2> Central chemoreceptors are activated. <3> The central chemoreceptors are depressed. <4> The arterial chemoreceptors are activated. <5> It produces loss of consciousness.
Output (English): 4