Bison

Dinner tonight was bison burgers with leftover slaw from yesterday. The burgers were made with organic bison from Dad's and sandwiched in a bun with grilled red onion, old cheddar, tomatos and chipotle mayo. I grilled them (on an indoor electric grill) for about 7 minutes and they turned out mouth-watering-ly (?) tender. We couldn't identify if the tenderness was due to

a) short grilling time
b) nature of bison meat
c) nature of organic meat
d) all of the above