78. LED Selection & Failure

Question.5

A product uses a 5 mm red LED driven at 20 mA continuously.
After 6 months of operation, users report:

The LED still works
But it is noticeably dimmer than before

Nothing changed.
What is the most likely cause?

The resistor value increased over time
The LED lens became dirty
Lumen degradation due to thermal stress
The LED has a lower Life- few months.
The LED polarity partially reversed

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