Answer the following question: Fact 1: Energy is used for heating a building by a heating system. Fact 2: Passive solar thermal energy involves heating buildings using direct sunlight. Given the two facts above, a method of heating a building might be what?

Available choices:
(1). dangerous.
(2). Blizzards.
(3). Heated arguments.
(4). threatened.
(5). major threat to health.
(6). Direct sunlight..
(7). Mothballs.
(8). slow and inefficient.
Answer:
(6).