challenge: What is used to derive the phase error signals in a double-ended synchronization control scheme?
  
  A. Phase of the incoming digital signal and the phase of the internal clocks at both exchanges
  B. Frequency of the internal clocks at both exchanges
  C. Voltage of the internal clocks at one exchange and the incoming digital signal
  D. Amplitude of the incoming digital signal and internal clocks at one exchange
  Answer: 