以下各段叙述给出了5个计算机专业英文术语的英文解释。这些英文专业术语依顺序分别是 (1) 、 (2) 、 (3) 、 (4) 和 (5) 。
Messages that are sent electronically over a network.
A series of instructions (commands) performed in a sequence specifying actions to accomplish a task.
The character code built into most modem personal computers.
The minimal unit of information，it can have the value 1 or 0.
The brain of a computer, it contains the ALU, the clock, many registers, the bus interface and circuitry that makes these parts work together.
4（）
A．byte
B．bit
C．logic
D．true
则z=x-y的最大值为（）。