Which sentence demonstrates the first conditional?

If it had rained, we would have stayed indoors.

If it rained, we stayed indoors.

If it rains, we would stay indoors.

If it rains, we will stay indoors.

If it rains, we will stay indoors.

The first conditional expresses a real possibility in the future.

It using the present simple tense in the "if" clause and "will" in the main clause.