Combine the two sentences into one in such a way that it contains the word given in the brackets

Combine the two sentences into one in such a way that it contains the word given in the brackets:
1. I wanted to thank them for the weekend, so I wrote them a letter. (TO)
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2. We got lost because we didn't have a map. (IF)
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3. He has a headache. He has to take an aspirin. (BECAUSE)
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4. Jane likes reading books. It interest her. (IN)
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