静息状态下脑平均耗氧量相当于全身的（）

A.25%

B.20%

C.15%

D.10%

E.8%

There is nothing in this world constant but inconstancy. —SWIFT Project after project designs a set of algorithms and then plunges into construction of customer-deliverable software on a schedule that demands delivery of the first thing built. In most projects, the first system built is 1. ______ usable. It may be too slow, too big, awkward to use, or all three. There is no 2.______ but to start again, smarting but smarter, and build a redesigned version in which these problems are solved. The discard and 3.______ may be done in one lump, or it may be done piece-by-piece. But all large-system experience shows that it will be done. Where a new system concept or new technology is used, one has to build a system to throw away, for even the best planning is not so omniscient (全知的) as to get it right the first time. The management question, therefore, is not whether to build a pilot system and throw it away. You will do that. The only question is whether to plan in advance to build a 4.______, or to promise to deliver the throwaway to customers. Seen this way, the answer is much clearer. Delivering that throwaway to customers buys time, but it does SO only at the 5.______ of agony (极大痛苦) for the user, distraction for the builders while they do the redesign, and a bad reputation for the product that the best redesign will find hard to live down. Hence plan to throw one away; you will, anyhow.

5.（）

A．worth

B．value

C．cost

D．invaluable