First of all, you should always follow the directions given for the world limit ie NO MORE THAN TWO WORDS.
If this was the case, your first answer – water and oxygen – would be incorrect. Your second answer, if it was given as exactly “early 1920s” in the text, would most likely be correct. When in doubt, write fewer words.
Hope this helps. If you copy and paste the particular questions, I can give you more specific help.