A caste system differs from a class system because:
A. a caste system is an open system of stratification, while a class system is a closed system of stratification.
B. it is not possible for an individual to change his or her caste, while a class system allows people to change their class through individual achievement.
C. the social mobility in caste systems varies from society to society, while in a class system there is no social mobility.
D. a caste system is a less rigid form of social stratification, while a class system is a comparatively more rigid form of social stratification.