This Linguaphone Advanced English Course is intended for students ________ already, have a good command of the basic grammar of English and are familiar with a reasonable wide selection of vocabulary.
句意：灵格风高级英语课程是为那些已经掌握了英语基本语法并且熟悉大量词汇的学生准备的。此句中空格处应填入关系词引导后面的定语从句修饰先行词“students”，C选项符合题意。故选C。