A cover letter is your chance to explain anything you couldn't in your resume (for fear of being too verbose).
So, if they want specific...give them specific. I'd also recommend making a few comments that show that you know what their company does... i.e. if they're building up their R&D dept. for blah blah blah then you say, "I like R&D in the blah blah blah field."
Hope that helps.
