Discussion
Why is the lamp current kept low?
Answer: B
Running the lamp hard produces a dense cloud of ground-state atoms in front of the cathode, which absorb the centre of the emitted line and broaden it - self-absorption. A broadened source line no longer matches the narrow atomic absorption and sensitivity falls. Lamp life does suffer too, which is a real but secondary consideration. Whenever a source and an absorber are the same element, the source's own vapour is a hazard worth designing against. Framing the question as which quantity the change acts upon makes the answer follow without calculation.
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