Software design is a (46) process. It requires a certain amount of fl air on the part of the (47) . Design can not be learned from a book . It must be practiced and learnt by experience and study of existing systems. A well (48) software system is straightforward to implement and maintain , easily (49) and reliable. Badly (48) software systems, although they may work are (50) to be expensive to maintain , difficult to test and unreliable.

A．maintain

B．maintains

C．maintaining

D．maintained

怎样用离子选择性电极测定离子浓度？简答测定步骤。