请教怎么做？
１ The operation ? is defined for all nonzero numbers a and b by a ? b = a/b – b/a. If x and y are nonzero numbers, which of the following statements must be true?
I. x ? xy = x(1 ? y)
II. x ? y = -(y ? x)
III. 1/x ? 1/y = y ? x
A. I only
B. II only
C. III only
D. I and II
E. II and III
２ On a certain transatlantic crossing, 20 percent of a ship’s passengers held round-trip tickets and also took their cars abroad the ship. If 60 percent of the passengers with round-trip tickets did not take their cars abroad the ship, what percent of the ship’s passengers held round-trip tickets?
A. 33 1/3%
B. 40%
C. 50%
D. 60%
E. 66 2/3% | 
