Whether something is in, on, at, or across another thing is a surprisingly tricky matter! There are some basic correct forms you can define, but after that there are few rules. It takes years of hearing English, reading English, and speaking English to really absorb all the correct forms for prepositions.
You can be in a room or in a book. You can be on a table or on television. You can be at the door or at the party. You can be across the road or across the topic.
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