【正确答案】 1、D    

【答案解析】[详细解答] E-mail gives learners an additional context for discussion that can be--but does not necessarily have to be- linked to topics being covered in class．[D]说E-mail让学习者可以讨论课内的也可以是课外的内容，而本段标题意为扩展课堂涉及的内容。所以答案为[D]。