Accretion of matter from the solar nebula eventually formed the earth. The different atmospheres and ocean were probably created as a result of the volcanic outgassing that initially occurred on earth.
I guess the answer is A. Zaire
"The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya.<span> <span>Wikipedia."
https://www.google.com/search?q=He+Silurian+period+occurred&rlz=1C1RUCY_enUS760US760&oq=He+Silurian+...
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