Stack is quite simple. Many computer systems have stacks built into their circuitry. They also have machine-level instructions to operate the hardware stack. Stack is ______ in computer systems.

A．useless

B．not important

C．simple but important

D．too simple to be useful

B处地形区为（　　）

A．伊朗高原

B．恒河平原

C．德干高原

D．印度河平原