Many of the activities performed during the preliminary investigation are still being conducted in（1）,but in much greater depth than before. During this phase, the analyst must become fully aware of the（2）and must develop enough knowledge about the（3）and the existing systems to enable an effective solution to be proposed and implemented. Besides the（4）for process and data of current system, the deliverable from this phase also includes the（5）for the proposed system.

A. hardware and software specification

B. system performance specification

C. formal requirements definition

D. general problem statement

凯撒密码是一种置换密码，对其破译的最多尝试次数是______。

A．2次

B．13次

C．25次

D．26次