Multiple choice:In the following keywords groups, which one is not all concerned

In the following keywords groups, which one is not all concerned with the search engine. @[C](3)
A. inverted file index, stop words, precision
B. word stemming, hashing, compression
C. distributed index, backtracking, query
D. posting list, thresholding, recall




A.inverted file index, stop words, precision
B.word stemming, hashing, compression
C.distributed index, backtracking, query
D.posting list, thresholding, recall


answer：C