The Nurse Documents a Patient's Radial Pulse Rate as 100

The nurse documents a patient's radial pulse rate as 100 beats/min and regular. One hour later, the nurse rechecks the pulse, and it is irregular at 120 beats/min. What is the most appropriate nursing action?

A) Ask another nurse to check the pulse.
B) Administer fluids while the patient is in bed.
C) Place the patient on a cardiac monitor.
D) Check the pulse in the opposite arm.