Which abstract data types the most appropriate for the below scenario?

Microzon is a cloud computing service looking to do user research. The company has some users take a satisfaction survey with a large amount of questions. Different users have different number of questions based on a variety of factors. A user gives a numerical rating in each of the questions. A user does not have to answer all questions, and can choose to leave some questions blank. A single user's ratings for the questions are recorded in a data structure. Any blanks are omitted, so all of the data is contiguous. Microzon collects the data from this structure, then clears the data structure. The data structure is now ready for the next user who takes the survey. What data structure should Microzon use to record a user$$s answers?