Read the passage.
excerpt from "Marie Curie and the Discovery of Radioactivity" by Mara Rockliff
The energy appeared to come from inside the metal itself, deep down at the level of its atoms. Marie Curie named this atomic energy radioactivity. A substance that emitted this energy was called radioactive.
Her experiments yielded a second surprise. A sample of uranium-rich pitchblende turned out to be three or four times as radioactive as pure uranium. How could this be? Marie had already tested every known element of pitchblende. None was radioactive. But something in that pitchblende, something besides the uranium, was sending out rays.
Question
It could be inferred that the word emitted in the first paragraph means "released."
Which context clue best helps confirm this meaning of the word emitted?
Responses
"sending out rays"
"turned out to be"
"deep down at the level of its atoms"
"three or four times as radioactive"