Final answer:
Justin should pick the black shirt because it absorbs more sunlight and converts it into heat, keeping him warmer.
Step-by-step explanation:
To stay warm in a cold environment, Justin should pick the black shirt because it will absorb more sunlight. When sunlight is absorbed by an object, it gets converted into heat. The black color absorbs more light and heat compared to the white shirt, which reflects most of the sunlight. Therefore, wearing the black shirt would keep Justin warmer.
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