After reading the paragraphs below, answer the questions that follow.
Electrocardiographs (ECGs or EKGs) provide data about the electrical activity of the heart. By placing a series of "leads" on a person's chest, the electrical activity of the heart can be recorded and analyzed in order to detect irregular heartbeats (arrhythmias) and other conditions. The figure below shows a normal EKG tracing for one cardiac cycle.
![](data:image/png;base64, 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The P region represents electrical activity spreading through the atria. The QRS region represents electrical activity spreading through the ventricles. The T region represents electrical activity in the ventricles returning to rest.
What is happening in the heart during the QRS region?
◦ The ventricles are contracting.
◦ The atria are contracting.
◦ The atria are filling with blood.
◦ The ventricles are filling with blood.