If x-y=8, which of the following must be true?
I. If x is negative, y must be negative
II. If x is positive, y must be positive
III. Both x and y are positive Choose one:  A) I only  B) II only  C) III only  D) I and II  E) II and III
The best way to approach such questions is to plug in values for x and y   Given: x-y=8   III. Both x and y are positive:   Let x=10 and y=2   x-y=8   But,   Let x=6 and y=-2   x-y=8   Therefore, NOT TRUE   II. If x is positive, y must be positive   Let x=10 and y=2   x-y=8   But,   Let x = 6 and y=-2   x-y=8   Therefore, NOT TRUE   I. If x is negative, y must be negative   If x is negative, for the expression x-y=8 to be true, y must be a -ve number. Otherwise, the sum of two negative numbers will yield another negative number!   Therefore, TRUE   I onlyns: 'I only'
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If x-y=8, which of the following must be true?
I. If x is negative, y must be negative
II. If x is positive, y must be positive
III. Both x and y are positive