In software engineering and systems engineering,（）is a description of a system's behavior as it responds to a request that originates from outside of that system.

A．black box

B．business rule

C．use case

D．traceability matrix

下列关于软土的叙述，正确的是（）

A.粒度成分主要为黏粒及粉粒

B.常有一定量的有机质

C.具有典型的海绵状或蜂窝状结构

D.常具有层理构造