I saw Sienna sprinting down the final stretch in long, graceful strides. The cheetah chased her prey, which, in this case, was a first-place ribbon.
Obviously Sienna is running and being compared to a cheetah, she was running swiftly.
That leaves us with either Choice B or Choice D.
The dialogue states that Sienna was "sprinting" or rather, "chasing" her pray "which, in this case, was a first-place ribbon."
Sienna was not racing against a fast animal -- the text was saying that she was <em>as fast </em>as an animal. Therefore, Choice B is incorrect.
That leaves us with Choice D, the correct answer. Sienna ran swiftly with the purpose of winning the first-place ribbon.
