单选题 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 (73) in computer systems.
A. useless B. not important
C. simple but important D. too simple to be useful

【正确答案】 C

【答案解析】[解析]
本题含义可表述为，栈是相当简单的，后两句从意思上看是计算机如何实现栈，实际说栈是计算机组成的一个必要的部分，最后一句是对前面三句的归纳。所以应该选择 C(简单并重要的)。