I am having difficulty designing a database, it seems straightforward enough, but i'm having issues, any help would be appreciated.

The basic concepts are:
company
promotion
quiz
question
customer

every company has more than one promotion
every promotion can only have 0 or one quiz
every quiz can have many questions.

The problem i have is how to relate customers (and their answers) to the quiz/questions.

A customer may only play a promotions quiz once.
(the start and end time of when the customer layed the quiz also need to be recorded)

many customers can play a promotions quiz