An air traffic controller issues the following instruction: 'Speedbird 235, descend to flight level two four zero, turn right heading one eight zero, reduce speed to two five zero knots.' The pilot reads back: 'Descend flight level two four zero, turn right heading one eight zero, reduce speed two three zero knots, Speedbird 235.' Which part of the instruction did the pilot read back incorrectly?
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A
The flight level
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B
The heading
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C
The speed
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D
The callsign