||Quizzes. Each quiz has a unique key (qkey); each row represents a single quiz.
||Each problem may have multiple "rows" of answers, e.g. a three-choice multiple-choice problem has 3 rows which define the possible answers.
||Maps sets of problems to a quiz. This tells us which quizzes "own" or "contain" which problems. Each row relates (connects) one problem to one quiz.
||Each problem has a unique key (pkey); each row represents a single problem.
||Records Takers' scores on problems. One row per Taker/problem/quiz/session combination. (Allows for a single problem to be shared across multiple quizzes, and also for taking a quiz more than once.) .