question: How is wavelength division multiplexing (WDM) implemented in optical fiber systems? option 1: Using light-emitting diodes (LEDs) and multimode fiber option 2: Using injection laser sources and multimode fiber option 3: Using light-emitting diodes (LEDs) and single-mode fiber option 4: Using injection laser sources and single-mode fiber option 5: Using frequency-shift keying (FSK) modulation 