In gel electrophoresis,the separated $DNA$ fragments are visualized after staining the $DNA$ with $...A...$ followed by exposure to $...B...$. Here $A$ and $B$ refer to:

• A
  $(A)-B-galactosidase, (B)-Infrared radiation$
• B
  $(A)-Ethidium bromide, (B)-UV radiation$
• C
  $(A)-Ethidium nitrate, (B)-\gamma-rays$
• D
  $(A)-Ethidium chloride, (B)-Radiowave$