The Challenger explosion and 9/11.

Angela is right. According to guidedogsof america

Breeds and Matching Process. Breeds Used: We use 70% Labrador Retrievers, 15% Golden Retrievers, and 15% German Shepherds. TheLabrador Retrievers make up the majority of our dogs because we have found them to be the most successful breed used for guide dogs

I'm not exactly sure but if I was to guess I'd say Labrador Retrievers.