Two identical resistors, each of resistance R, are connected in series to a dc source. A voltmeter of internal resistance Rv is placed across the second resistor, and its reading is found to be 80% of the true potential difference that existed across that resistor before the voltmeter was connected. Which of the following gives the required value of Rv?
a) 32R
b) 2R
c) 31R
d) 4R