请教一个英语中逗号的问题

请教大家一个关于逗号的问题，是OG小鱼版中第37题：

37. While some academicians believe that business ethics should be integrated into every business course, others say that students will take ethics seriously only if it would be taught as a separately required course.

(A)  only if it would be taught as a separately required course

(B)   only if it is taught as a separate, required course

(C)  if it is taught only as a course required separately

(D)  if it was taught only as a separate and required
                course

(E)   if it would only be taught as a required course,
                separately

答案中 a separate, required course不是很理解，两个形容词修饰一个名词我们见的不少，比如a tall and heave man等等，为什么要加逗号呢？而且我看了语法中逗号的用法，当三个以上的形容词修饰的时候才可以用逗号分开。还请高人指点一下。谢谢了。

没有逗号也是对的,But "a separate required course" can be understood in two possible meanings.

1). "required course" as a unit; separate modifies "required course" 

2).  "a separate course and a required course". 

However, a separate, required course = a separate course + a required course.