This is the question:

Question 6 2 pts Let S be the set of all strings of O's and 1's, and define D:S -> Z as follows: For every s E S, D (s) = the number of 1's minus the number of O's in s. Which one of the following statements about D is true? D is 1-1 and onto. O D is not 1-1 but is onto. O D is not 1-1 and not onto. D is 1-1 but not onto