单项选择题
which of the following are equal以下几个量中相等的是:
① Numbers of different n-bit binary 不同的n位二进制数个数
② The number of legal parentheses matched by n pairs of parenthesesn对小括号所能构成的合法括号匹配个数
③ Numbers of different stack shuffle for {1, 2 .. n}{1, 2 .. n}的不同栈混洗个数
④ Number of operator stack push operations during infix expression evaluation with n operators含n个运算符的中缀表达式求值过程中运算符栈push操作的次数
A．①②
B．②③
C．③④
D．②④
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