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The subclasses, heavy and light, were only introduced for bombing aircraft towards the end of World War I when they had become more specialized. |
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As World War I was drawing to a close, bombing aircraft had become so specialized that they had to be divided into two new subclasses, heavy and light. |
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Bombing aircraft had become so much more specialized by the end of World War I that two new subclasses were recognized, light and heavy. |
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Towards the end of World War I, bombing aircraft were becoming more specialized and were divided into two subclasses, light and heavy. |
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As bombing aircraft were becoming more specialized towards the end of World War I, they had to be divided into two subclasses, heavy and light. |