Which of the following are characteristics of the normal ECG? * a. Waveforms and intervals that appear at regular intervals at a rate of 60 to 90 beats per minute (in the adult).
b. Upright and slightly asymmetrical P waves, each followed by a QRS complex of normal upright contour, duration, and configuration
c. A PR interval (PRI) with a duration of 0.12 to 0.24 seconds that precedes each QRS complex.
d. A flat ST segment followed by an inverted, symmetrical T wave.