Which Event of the Past Century Do You Think Will

Which event of the past century do you think will have announced our presence on Earth MOST effectively to extraterrestrial watchers?

A) the appearance of artificial satellites orbiting Earth after 1957
B) nuclear weapon explosions producing extremely intense but brief flashes of light and electromagnetic radiation
C) the slow changes in vegetation patterns and the appearance of manmade structures such as road systems and cities on Earth
D) the slow buildup of radio transmissions after the invention of radio, with modulated signals carrying sound and visual television images