Both X and Y plates of a CRO are excited by ac signals. The display is an ellipse which makes one rotation in 5 seconds. This means that

• A. the frequency of one input is 5 times that of other
• B. the difference between the frequencies of the two signals is one fifth of hertz
• C. the difference between the frequencies of the two signals is 5 Hz
• D. either (a) or (c)