None, because people who glow in the dark don't need light bulbs.
Note: Topical to the Chernobyl Reactor disaster of 1984.
or
None, because they were copycats
or
They can make little things count.
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Just one, but he/she will spend hours checking for technical problems.
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What's the use, they'd still have bear feet
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I was amazed, I never knew they worked.
or