Which of these events of the past 150 years announced human presence on Earth MOST effectively to extraterrestrial watchers?
A) appearance of artificial satellites orbiting Earth after 1957
B) nuclear weapon explosions producing extremely intense but brief flashes of light and electromagnetic radiation
C) slow changes in vegetation patterns and the appearance of humanmade structures such as road systems and cities on Earth
D) slow buildup of radio transmissions after the invention of radio, with modulated signals carrying sound and visual television images
Correct Answer:
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