- A drought is a kind of slow environmental change
- A drought occurs over a period of months / years
- Therefore, a drought is an environmental change that occurs over a period of months or years

Therefore, a drought is an environmental change that occurs over a period of months or years
[Question]Which of these environmental changes goes on for months or years?
[Context]- A drought is a kind of slow environmental change 
- A drought occurs over a period of months / years