The verb 'is' in this clause relates to the subject of 'a single task' and therefore must match it in number. 'Are' is incorrect. 'More than' is additional information, however, 'is' relates to 'a single task'.
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Stenka25there was none of "more than a single task are."That is not surprising. Whenever you see "more than one ..." or "more than a single ..." as the subject, use a singular verb. These kinds of expressions are always singular.