Galaxies were originally called "Spiral Nebulae". Their true nature was first appreciated when: a. ) Galileo disproved t

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Galaxies were originally called "Spiral Nebulae". Their true
nature was first appreciated when:
a. ) Galileo disproved the Geocentric Model
b. ) Newton developed his theory of gravity
c. ) Hubble measured the distance to the Andromeda Galaxy
d. ) The Cosmic Microwave Background was discovered
In recent decades, the level of Carbon Dioxide (CO2) in Earth's
atmosphere
a. ) has increased, but not by much compared to historical
records
b. ) is lower than 100 years ago
c. ) is difficult to measure reliably
d. ) has increased dramatically to a new high level
The oldest stars in our Galaxy can be found:
a. ) orbiting black holes
b. ) in the disk of the galaxy
c. ) in globular clusters
d. ) in open clusters