In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.

Question: What is cardiac output? 
 Options:  <1> The volume of blood that leaves the heart per unit of time (ml / min or l / min). <2> The complete heartbeat constituted by contraction (systole) and relaxation (diastole) in both atria and in both ventricles measured in time (sec). <3> The blood that remains in the heart when the pressure gradient determines the distension of the cardiac fibers. <4> The power gradient of the contraction, also called contractility. <5> The ventricular volume that remains constant.
1