I'll give you a question, please answer with step-by-step reasoning process.
John spent his days planting ficus trees. He did so because trees were an important part of what?\nOptions:\n- tropical rainforest\n- ecosphere\n- sunny window\n- pots\n- dirt

ecosphere
Trees are crucial part of our ecosphere for survive of humans and other live habitats on earth.

Planting trees is required to maintain the balance in ecosphere.

.

Therefore, the answer is ecosphere