A. be seen by the naked eye
B. not pass through an aluminum foil
C. be present naturally in our surroundings
D. be used as a tracer
Correct Answer: Option D
D. be used as a tracer
A. be seen by the naked eye
B. not pass through an aluminum foil
C. be present naturally in our surroundings
D. be used as a tracer
Correct Answer: Option D
D. be used as a tracer
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