根据短文回答 51～65 题。
Making the Leap
Jumps play a big role in many styles of dancing.Generally, what makes a jump impressive is its hang time,the amount of time a dancer spends in the air.
The quest for greater hang time is a battle against gravity, the constant
____________(51)pull of Earth,said Laws.To leave the ground at all.a dancer has to use leg muscles to create an upward push that is greater than Earth's downward pull.But the final____________ (52)of any jump depends on just one thing：the upward speed of the body just as the dancer leaves the ground.
Strengthening muscles so they can push harder is one obvious____________ (53)to achieve higher jumps and increase hang time.But ballet dancers also use a simple trick to gain the illusion of staying in the air longer without actually doing S0.In a huge sideways jump called a grand jeté(小跳),a____________(54)ballet dancer seems to float for an impossible length of time.Of course,a dancer can't really hang in the air.The laws of physics decree that during any jump,a dancer's center of gravity must follow a parabola(抛物线).A parabola is the same____________ (55)path a ball takes when you throw it into the air.So how do dancers make it look like they're hanging in the air？
A dancer____________ (56)the illusion of floating in the air by lifting her legs and arms as she approaches the peak of the jump.The____________ (57) of her body
responds by sinking a bit.If her timing is just right,she'll seem to float sideways,instead of rising and fallin9.The effect is not only beautiful；it____________ (58)makes the jump seem bigger by" stretching out" the peak.
Of course,what goes up must come down.During a typical grand jete,a dancer's center of gravity raises 2 feel____________(59)the ground。Pulled by gravity from such a height,the dancer's body falls very fast—roughly 3.4 meters per second—by the time it reaches the floor.
As it falls,the body carries with it momentum(动量).Momentum is the weight of the body multiplied by its____________ (60).The bigger the body is and the faster it falls,the greater its momentum.
The only way a dancer can stop dropping through the air is by stopping the body's momentum,which requires an____________ (61)force—the ground.Landing can____________ (62)injuries.The dancer can ease the landing by bending her knees and letting her arms fall。but she also gets help from an unexpected source：the floor.Wooden dance floors are designed to____________(63)like shock absorbers.They can recoil(回缩)as much as an inch under extreme pressure.That little bit of give(弹性)makes a big____________(64).Landing on a wooden floor, the dancer undergoes a slower change in momentum than she would hitting a rigid floor.The give in the floor allows the decrease in momentum to happen more____________ (65) with less force and less chance of injury.
第 51 题

A. side
B. upward
C. up
D. downward