the plants used to be harvested for food. the island used to be located farther south. the climate of the island used to be warmer. the fossils used to be more resistant to cold.
I gave my students this multiple choice question: Finding fossils of warm-weather plants in the rock layers of a cold northern island suggests that

Only one answer is correct among these 4 choices:
- the plants used to be harvested for food.
- the island used to be located farther south.
- the climate of the island used to be warmer.
- the fossils used to be more resistant to cold.

Could you tell me which one is correct?

the climate of the island used to be warmer.