Software entities are more complex for their size than perhaps any other humanconstruct, because no two parts are alike （at least above the statement level）. If they are, wemake the two similar parts into one, a（ 71 ）, open or closeD． In this respect software systemsdiffer profoundly from computers, buildings, or automobiles, where repeated elements abound．
Digital computers . are themselves more complex than most things people build;they have very large numbers of states This makes conceiving, describing, and testing them harD． Software systems have orders of magnitude more（ 72 ）than computers do
Likewise, a scaling-up of a software entity is not m erely a repetition of the same elementsm larger size; it is necessarily an mcrease in the number of different elements. In most cases, the elements interact with each other in some（ 73 ）fashion: and the complexity of the wholencreases much more than linearly.
The complexity of software is a（an）（ 74 ）property, not an accidental one Hencedescriptions of a software entity that abstract away its complexity often abstract away its essence.Mathematics and the physical sciences made great strides for threecenturies by constructingsimplified models of complex phenomena, deriving properties fiom the models, and verifyingthose properties experimentally. This worked becausethe complexities（ 75 ） in the modelswere not the .essential properties of the phenomena: It does not work when the complexities are the essence.
Many of the classical problems of developing software products derive from this essential complexi and its nonlinear uicreases with size. Not only .technical problems but management problems as well come from the coin plexity.
A． task
B．job
C．subroitune
D．Program
A．states
B．parts
C．conditions
D．Expressions
A．linear
B. nonlinear
C．Parallel
D．Addititive
A. surface
B． Outside
C．exterior
D．Essential
A． fixed
B． Included
C．ignored
D．stabilized