What is the main conflict in The City of Ember by Jeanne DuPrau?

Lina Mayfleet and Doon Harrow are both twelve years old. They are members of the Highest Class of the Ember School. As the book opens, it's Assignment Day or the day the twelve-year-olds are given their jobs. The assignments are arbitrary. There are jobs written on slips of paper in a bag and each student draws a slip to determine his job. Lina is assigned to work in the Pipeworks and Doon is assigned to be a messenger. Doon is certain that the city is in trouble and says so. He trades assignments with Lina, hoping he can do something good for the city if he's working in the Pipeworks. Lina accepts the trade, loving the idea of being a messenger.