Which one of the following statements is the MOST accurate?

a. Spot exchange rates are always higher than forward exchange rates.

b. Spot exchange rates are always lower than forward exchange rates.

c. Spot exchange rates and forward exchange rates are always equal.

d. Spot exchange rates and forward exchange rates are equal when the value date and the date of the spot transaction are the same.

e. Spot exchange rates and forward exchange rates never move closely together.