During a NATO flight exercise, the NATO command has many pilots speaking different languages and more or less trained on the different types of aircraft. The command had to form pilot/co-pilot pairs.

Which of the following best describes the challenge faced by the NATO command during the flight exercise?

a. Language barriers and varying levels of training among pilots
b. Formation of pilot/co-pilot pairs
c. Different types of aircraft used in the exercise
d. The complexity of the NATO flight exercise