得到授权的实体在需要时可访问数据，即攻击者不能占用所有的资源而阻碍授权者的工作，那么这里表述的是信息安全的(47)要素。

(86)

(84)

(85)

Electronic mail and filing permit a user to compose and transmit a message on an office(71)system. In the early 1970s, the ARPANET community developed a number of such systems which have been heavily used. Through standard message format(72), several hundred different computers and electronic mail interfaces are specification of how(73)and return addresses should appear on envelopes and which sizes are allowable for envelopes. In the electronic message world, they describe what(74)of characters are required at the(75)of a message to identify the sending and receiving mailboxes.

(83)

(81)

(80)

由固定分区方式发展到分页存储管理方式的主要推动力是(14)；由分页系统发展为分段系统，进而又发展为段页式系统的主要目的来源于(15)和(16)。

(78)

8层完全二叉树至少有(63)个结点，拥有100个结点的完全二叉树的最大层数为(64)。