The different waves of normal electrocardiogram (ECG) of a human subject are shown below :

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Components of a normal ECG tracings consist of waveform components which indicate electrical events during one heart beat. These waveforms are labeled P, Q, R, S, T and U. The first deflection is the P wave associated with right and left atrial depolarization. P wave is the first short upward movement of the ECG tracing. It indicates that the atria are contracting, pumping blood into the ventricles. Hence statement A is correct.
        The second wave is the QRS complex. Typically this complex has a series of 3 deflections that reflect the current associated with right and left ventricular depolarization. The QRS complex, normally beginning with a downward deflection, Q; a larger upwards deflection, a peak (R); and then a downwards S wave. The QRS complex represents ventricular depolarization and contraction. Hence statement C is also correct.
        T wave is normally a modest upwards waveform representing ventricular repolarization. Hence statement B is incorrect.
        The QT interval is measured from the beginning of the QRS to the end of the T wave. It represents the time in which the ventricles depolarize and repolarize and is a measure of ventricular action potential (AP) duration. Hence statement D is also incorrect.