E7F06 (D) 
Which of these choices best describes a crystal marker generator?
A. A low-stability oscillator that sweeps through a band of frequencies
B. An oscillator often used in aircraft to determine the craft's location relative to the inner and outer markers at airports
C. A crystal-controlled oscillator with an output frequency and amplitude that can be varied over a wide range
D. A crystal-controlled oscillator that generates a series of reference signals at known frequency intervals
