A famous explorer in search of a hidden treasure has been imprisoned by a magical demon. The demon listens to the explorer's objective and later secretly summoned the treasure by using his magical powers. Next day, the demon takes the explorer in front of three rooms.
The demon explains that:

1. Precisely one of the rooms contains the treasure and the other two rooms each contains a lion.
2. The sign on the room containing the treasure was true, and at least one of the other two signs was false.

Here are the three signs:

+----------+ +--------- -+ +----------+
|    I     | |    II     | |  III     |
| A LION   | |  A LION   | | A LION   |
| IS IN    | |  IS IN    | | IS IN    |
| ROOM II  | | THIS ROOM | | ROOM I   |
+----------+ +-----------+ +----------+

If the explorer chooses the room with the treasure, he is to be set free to leave with the bounty with demon's blessings.
But, if he chooses any of the rooms having a lion, the door will be closed from the outside as soon as he enters the room, and the lion will kill him.

Which room should the explorer choose? Provide adequate reasoning for your answer.