Just Before Winning the Belmont Stakes and Becoming the First

Just before winning the Belmont Stakes and becoming the first horse in 37 years to win the Triple Crown, the odds against racehorse #5 (named American Pharoah) winning the race were 3 to 5 (3:5 odds against winning) . What did the oddsmakers consider the probability that American Pharoah would win that year's Belmont Stakes?

A) 3/5
B) 5/3
C) 0.375
D) 0.625
E) 0.35