Final answer:
With 9 QRS complexes in a 6-second strip, the heart rate is 90 beats per minute since 9 multiplied by 10 gives 90 bpm.
Step-by-step explanation:
If you have 9 QRS complexes on a 6-second ECG strip, the heart rate (HR) can be calculated as follows: since there are 9 QRS complexes in 6 seconds, we multiply the number of complexes by 10 (since there are 10 six-second intervals in a minute) to get the HR per minute. Therefore, 9 QRS complexes multiplied by 10 equals 90 beats per minute (bpm). So, the correct answer is B. 90 bpm.