By "a system of built-in obsolescence" the author means ______.

A．no formal systems exist in modern industrial society

B．old people have no status in modern industrial society

C．young people have chances in modern industrial society

D．knowledge changes rapidly in modern industrial society

下图是山东沂蒙山区特有的一种地貌景观“岱崮地貌”景观图，过去在地貌学上称之为“方山”，该山岩石中普遍含有三叶虫化石。据此回答题。

该山区的岩石主要是（）

A.花岗岩

B.石灰岩

C.大理岩

D.玄武岩