Because the time was going fast
It's definitely D.
It's for sure not C. And I've never read the full story, but this passage doesn't indicate a depressing mood of foreshadowing danger. And the vocabulary in the writing doesn't impact the story itself.
Answer:
Gliders, like other aeroplanes, fly by using ailerons, rudders, and elevators. Gliders are equipped with flaps to control fall speeds by boosting lift and producing drag. Many modern gliders also have airbrakes or spoilers, which obstruct airflow across the wing, raising drag and lowering lift.
