Final answer:
The prepositions to be used are 'through,' 'down,' 'among,' 'into,' and 'across.' The extra preposition that is not needed is 'over.'
Step-by-step explanation:
Let's complete the sentences with the appropriate prepositions:
- We walked through the forest in the snow – it was beautiful.
- The mountain was very high, so I didn’t look down.
- It's very hot, so let’s sit over there among those trees.
- When we went into the cave, it was very dark!
- Alice took a boat across the lake because she was staying on the other side.
The extra preposition that was not used is 'over.'