Which question is being asked in this story problem?

martin has a rock collection and a book collection. he has a total of 60 rocks and books combined in his collection. he has 24 rocks in his collection.

how many books are in martin's collection?

a.
how many rocks does martin have in his collection?

b.
how many items does martin have in his rock collection and book collection combined?

c.
how many more books than rocks does martin have in his collection?

d.
what is the total number of books in martin's collection?