A mutual fund manager wants to create a fund based on a high-grade corporate bond index. She first distinguishers between utility binds and industrial bonds; she then, for each segment, defines maturity intervals of less than 5 years, and greater than 10 years. For each segment and maturity level, she classifies the bonds as callable or non-callable. For the manager’s sample, which of the following best describes to:sampling approach number of sampling cells（）
A. simple random sample 3
B. simple random sample 12
C. stratified random sample 12