A project has 4 phases. Phase 1 takes 3 weeks, Phase 2 takes twice as long as Phase 1, Phase 3 takes half as long as Phase 2, and Phase 4 takes as long as Phase 1 and Phase 3 combined. How many weeks does the entire project take?
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A
15 weeks
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B
18 weeks
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C
21 weeks
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D
24 weeks