A mutual fund manager wants to create a fund based on a high-grade corporate bond index. She first distinguishes between utility bonds and industrial bonds; she then, for each segment, defines maturity intervals of lee than 5 years, 5 to 10 years, and greater than 10 years. For each segment and maturity level, she classifies the bonds as callable or noncallable. She then selects bonds from each of the subpopulations she has created. For the manager’s sample, which of the following best describes the sampling approach（）

A. Systematic sampling.

B. Simple random sampling.

C. Stratified random sampling.

14世纪意大利盲人音乐家（）创造了一种（）是13~15世纪六度到八度终止式的变化处理。