The best way would be DNA
Use process of elimination. From what you gave me to read, you can cross off competition. A drought killed the plants and not competition to live. Go down the list of words and look up the definition. Look at nudation and ecesis later because they are different words that may or may not be challenging to understand. You know what reaction and migration is without looking it up. Once you cross off all words answers that don't match up, ask yourself, which remaining answer goes? Nudation is basically new plants that have successfully started growing after a huge environmental change. Ecesis is sort of similar however, the establishment in a new habitat or environment is not from a big change. Therefore, nudation is the correct answer.
Your answer for this problem is going to be :T-A-A-G-U-T-U-G
A, because it the constant variable
Answer: Earth spins on its axis
hope this helps (:
