Which of the following represents a true distinction between cross-sectional and longitudinal ...

Which of the following represents a true distinction between cross-sectional and longitudinal designs?
 
  a) Cross-sectional designs can be used to compare differences across cohorts, while longitudinal designs can be used to compare differences within cohorts.
   b) Cross-sectional designs are sensitive to changes in methodology, while longitudinal designs are relatively impervious to changes in methodology.
   c) Cross-sectional designs study the same individuals multiple times, while longitudinal designs study different individuals each time.
   d) Cross-sectional designs are prone to practice effects, while longitudinal designs are not subject to practice effects.