How can I select the lot number of the assemply from the components fro a job in wip

I have item AB is a combine of purchasable and stockable Item A and purchasable and stockable Item B
Create in BOM for item AB as item a and item B
Item A has a lot expire on 20-sep - 2013
item B has a lot expire on 30 apr - 2014
Create a job for item AB
AB is a combine of item A and item B
What will be the lot for item AB
I need to to track lot for both item A and Item B
How can I selectt the lot number form the components

I need the expire date of the new item AB is 20 - sep-2013