Have a look at the second paragraph,
Setting the scene, of my article
D3.js crash course - Part2[
^] - loading jQuery this way ensures that you load it only once, and that you are in control of the version that gets loaded.
For a web application that is supposed to run on the intranet I would install jQuery as a part of the web application, for an internett application I would use a CDN like "ajax.googleapis.com"
Best regards
Espen Harlinn