How is the rate determined on an ECG when the ventricular heart rate is irregular?

A) Count the number of QRS complexes in a 2 vertical marks then multiply by 6
B) Count the number of QRS complexes in a 2 vertical marks then multiply by 10
C) Count the number of QRS complexes in 3 vertical marks then multiply by 10
D) Count the number of QRS complexes in a 3 vertical marks then multiply by 6