5 Fill in: in, at, on. 1 We have an environmental awareness day at my school. 2 Don't leave the light on when you're not in the room. 3 The enclosure is five meters in length. 4 We spent hours gazing at the pretty stars in the night sky, 5 Sadly, many people have difficulty in practicing the three Rs: reduce, recycle, reuse. 6 On a compost heap, you put new material on top of older material.