The course normally attracts 20 students per year,------- up

The course normally attracts 20 students per year,_______ up to half will be from overseas.
选项:a、in which b、 for whom c、 with which d、 of whom
选哪个?为什么?请 翻译
选D
这个课程通常每年吸引20名学生（参加）,在他们中间有半数来自海外.
of whom 就是指代在他们中间 of 20 students