Which of the following is correct for optical amplifiers and regenerators? (Multiple choice)

Which of the following is correct for optical amplifiers and regenerators? (Multiple choice)
A . Higher regenerator cost
B. When the regenerator works at a specific bit rate and a specific frame format
C. Where the amplifier can work at any bit rate and frame format
D. Optical amplifiers are rarely used, and regenerators are commonly used

Answer: ABC