The technique of imitation or imitative counterpoint

The technique of imitation or imitative counterpoint
 
  a. is a type of counterpoint in which one voice introduces each new theme and is answered by other voices that enter in succession as it continues to sing.
  b. is a type of counterpoint in which one voice introduces each new theme and then drops out completely as it is answered in succession by other voices.
  c. was only used briefly during the Renaissance era, and was not well accepted.
  d. is a rhythmic device that enables the singers to stay together during long polyphonic works.