How to merge files?

Often I need to add data from one spreadsheet to the appropriate places on another spreadsheet. For example:

Sheet A has 10,000 records with these fields: id#, name, address, place of employment.

Sheet B has 5,000 records these fields: id#, GPA, college major, type of degree.

Some of the records in B contain information for the same id#'s as sheet A. I want to add this information together so that a Sheet C will have these fields: id#, name, address, place of employment, GPA, college major, type of degree.

What's the easiest way for Excel to do this?