In the opinion of Li Ming, a science student in that high school, physics is_______ chemistry, but much easier than English.

A．as difficult a subject as

B．as a difficult subject as

C．a subject so difficult as

D．difficult as a subject as

程序的并发执行是现代操作系统的基本特征之一，为了更好地描述这一特征而引入了______ 这一概念。