Which sentence uses dashes correctly?

a. "i would not step there if i were—! " she began as i slipped on the wet floor.

b. several birds stayed at the feeder—hoping for a good meal.

c. turquoise beads and—a silver medallion—adorn her favorite necklace.

d. my sister—a big fan of the outdoors—walks two miles—every afternoon.

(leaning more towards a., though i'm not sure.)