Software design is a (66) process. It requires a certain (67) of flair on the part of the designer. Design can net be learned from a book. It must be practiced and learnt by experience and study of existing systems. A well (68) software system is straight forward to implement and maintain, easily (69) and reliable. Badly (68) software systems, although they may work are (70) to be expensive to maintain, difficult to test and unreliable.

69（）

A．understand

B．understands

C．understanding

D．understood

网络学习空间的模型结构包括哪几层（）

A.用户层

B.控制层

C.虚拟资源层

D.物理层