Prototype method is one of the software engineering methods frequently used, which of the following item is the necessary condition for implementing prototype method Ⅰ．Can acquire the user requirement quickly Ⅱ．Have the tools for building software fast Ⅲ．Have a certain accumulation of prototype systems

A． Ⅰ only

B． Ⅰand Ⅱ

C． Ⅱ and Ⅲ

D． all