Wordpress - Conditional Statement - Query Category and display content on single post page.

I need some conditonal php code to show specific content on the single post template based on the category. Basically, I want to display an advertisement based on category. **The single post is listed under multiple categories so the code needs to perform a query to see what category the site visitor clicked on to arrive at that page and then display the correct ad for that category.** There are 6 main level categories which will create the ad zones. Any child categories will need to show the ad of the main category. For example a postA is under cat1> Subcat2 and also under Cat3 > subcat5 If the site visitor clicks on CAt1 subcat2 they will get the cat1 advertisement and if they click on Cat3 subcat5 they will get the cat3 ad. **Note:** the code should also be able to be used on the sidebar to perform the same function for two more ad spots. ## Deliverables You should be familiar with Wordpress. The solution must work with the multiple categories. Please let me know any questions! Thanks.