Answer :

After the oil boom that happened in Pennsylvania during 1859. 3 other states started to discovered vast oil reserved in their country.
=> 2 of these countries are California and Texas.
=> For Texas, it was a great opportunity to be able to discover vast oil reserves for their country because it helped a lot with their economic growth. During 1940, Texas was already considered as the one of the biggest oil producing state.
Later they were joined by Kansas, Oklahoma and Illinois.





TSO
Two of those states were Texas, and California. A few other states also discovered oil, such as Kansas, Ohio, and Oklahoma.

Hope that helped :)