Task: Question Answering
Input (English): Question: According to the Frank-Starling mechanism of the heart, under physiological conditions the cardiac output is directly proportional to: 
 Options:  <1> The size of the ventricle. <2> The heart rate <3> The venous return. <4> The thickness of the myocardium. <5> All the previous answers are correct.
Output (English): 3