Learned as he was, the professor found it hard to get his main ideas ( ) to his students.A. overB. acrossC. roundD. on

本题考查知识点：固定搭配的用法。
解题思路：本题需要判断哪个介词能与“get...to...”构成合适的固定搭配来表达“让学生理解主要观点”这一含义。我们需要对每个选项所构成的搭配进行分析，看其语义是否符合语境。
解析：

• A选项：“get...over”常见的意思有“克服；恢复；熬过”等。例如：“You should get over your fear of speaking in public.”（你应该克服当众讲话的恐惧。）在本题中，“get his main ideas over to his students”语义不通，不能表达“让学生理解主要观点”的意思，所以A选项不正确。
• B选项：“get...across to...”是一个固定搭配，意为“使……被理解；把……讲清楚给……听”。在本题中，“the professor found it hard to get his main ideas across to his students”表示“教授发现很难让他的学生理解他的主要观点”，语义符合语境，所以B选项正确。
• C选项：“get...round”有“说服；使传播；绕行”等意思。例如：“I can't get him round to my way of thinking.”（我无法说服他接受我的想法。）“get his main ideas round to his students”不符合表达习惯，不能准确表达“让学生理解主要观点”，所以C选项不正确。
• D选项：“get...on”常见的意思有“进展；继续；穿上”等。例如：“How are you getting on with your work?”（你的工作进展如何？）“get his main ideas on to his students”语义不恰当，不能表达“让学生理解主要观点”，所以D选项不正确。