Which of the following statements is correct?

a. Secondary file organization methods make insertion and deletion of records a lot easier.

b. An important application of tree data structures is when they provide a physical index structure to speed up retrieval of records.

c. An advantage of one-way linked lists, where each record contains a pointer to its logical successor, is that not all records must be retrieved any longer, because frequently only part of the list needs to be processed.

d. An inverted file defines an index over a unique, ordered search key of the dataset.