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University of Bristol Faculty of Arts: Improve Your Writing: Using the Colon
In this exercise, students are given 3 sentences from which they must select the one with the correct usage of the colon. There is also a link for other grammar exercises.
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University of Bristol Faculty of Arts: Using the Colon to Improve Style
In this exercise, students are given sentences without a colon; they are asked to retype each sentence using the colon to make it more forceful. The site also offers a link with more grammar exercises.