Choose the word or set of words that, when inserted in the sentence, best fits the meaning of the sentence as a whole. 

Scientists are studying the birth and growth of thunderstorms to discover what causes the difference between showers that enable crops to ------- and ------- storms that cause floods and erosions.

1.   flourish . . violent
2.   wither . . damaging
3.   grow . . harmless
4.   parch . . severe
5.   multiply . . essential