The purpose of using ultrasonic scanning was _____

单项选择题

Is athletic expertise attained or innate Those who have suffered the tongue-lashing of a cruel games master at school might be forgiven for doubting the idea that anyone and everyone is capable of great sporting achievement, if only they would put enough effort into it. Practice may make perfect, but not all are built in ways that make it worth bothering in the first place.
The latest evidence of this truth has been gathered by Sabrina Lee of Simon Fraser University in Vancouver and Stephen Piazza at Pennsylvania State University. They have looked at the physical structure of short-distance runners and found that their feet are built differently from those of couch potatoes.
Dr. Lee and Dr. Piazza already knew that short-distance runners tend to have a higher proportion of fast- contracting muscle fibres in their legs than more sedentary folk can muster. They suspected, though, that they would find differences in the bone structure as well. And they did.
They looked at seven university sprinters who specialize in the 100-metre dash and five 200-metre specialists, and compared them with 12 non-athletic university students of the same height. In particular, they looked at the sizes of bones of the toes and heel. They also used ultrasonic scanning to measure the sliding motion of the Achilles tendons of their volunteers as their feet moved up and down. This allowed them to study the length of the lever created by the tendon as it pulls on the back of the heel to make the foot flex and push off the ground.
Dr. Lee and Dr. Piazza found that the toes of their short-distance runners averaged 8.2cm in length, while those of common people averaged 7.3cm. The length of the lever of bone that the Achilles tendon pulls on also differed, being a quarter shorter in short-distance runners.
These findings suggest short-distance runners get better contact with the ground by having longer toes. That makes sense, as it creates a firmer platform to push against. In a short-distance running race, acceleration off the block is everything. Cheetahs, the champion of short-distance runners of the animal kingdom, have non-flexible claws that give a similar advantage.
It is possible—just—that the differences in physical structure are the result of long and rigorous training. But it is unlikely. Far more probable is that the old saying of coaches, that great short-distance runners are born not made, is true. Everyone else, games masters included, should just get used to the idea.

The purpose of using ultrasonic scanning was ______.

A．to measure the frequency of the Achilles tendons" pulling as the feet moved
B．to see how the Achilles tendons slide as the feet moved up and down
C．to compare the sliding motion of the Achilles tendons with the pulling of muscle fibres in the legs
D．to measure the length of the lever of bone that the Achilles tendon pulls on