Which of the following statements is true about two wattmeter method for power measurement in three phase current?

A. Power can be measured using two wattmeter method only for star connected three phase circuits

B. When two meters show identical readings, the power factor is 0.5

C. When power factor is unity, one of the wattmeter reads zero

D. When the readings of the two wattmeters are equal but of opposite sign, the power factor is zero

Answer: Option D