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Which statement appropriately describes the content and purpose of CO domain in SDTM?

  A special purpose domain that includes a set of essential standard variables that describe each subject in a clinical study

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  A special purpose domain that contains comments that may be collected alongside other data

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  A special purpose domain that contains the actual order of elements followed by the subject, together with the start date/time and end date/time for each element

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  A special purpose domain that contains the actual start and end date/time for each visit of each individual subject

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