In fall 2006，the National Basketball Association （NBA）started using basketballs made with synthetic ，or man-made ，material instead of leather（皮革）．They made the change be- cause they wanted every basketball they use to feel and bounce（弹起）the same．Not all leather balls are exactly alike in weight or how they bounce，but the synthetic balls are．
However，some players complained right away that the new balls bounced differently and were actually harder to control than the leather ones．
Physics helped answer questions about why the new balls felt different．For example，the scientists studied friction（摩擦力），which in this case affects the ability of a player to hold onto a ball．"The greater the friction，the better it will stick to his hand，"explains a scientist．
Tests on both wet and dry balls showed that while the synthetic ball was easier to hold onto when dry，it had less friction and became much harder to hold onto when wet．That's because sweat stays on the surface of the synthetic balls but gets taken into the leather balls．
The scientists also tested bounce and found that the logo （商标）printed on the new balls made their surface uneven（不平） and caused them to bounce a little strangely com- pared with the leather balls．
In January，the NBA went back to using the leather balls．They aren't perfect，but for now，that's just the way the ball bounces．
Why were the synthetic balls expected to be used in NBA according to the text?

A．Because they are easier to control．

B．Because they waste less material．

C．Because they look exactly alike．

D．Because they feel and bounce alike．

The ball is easier to hold on to_________

A．when friction is greater

B．when friction is less

C．when it is heavier

D．when it bounces strangely

The synthetic ball bounces differently from the leather one mainly because _________

A．it is wet	B．it is dry
C．there's a logo on it	D．its weight remains the same

The underlined word sweat refers to ____________ .

A．灰尘

B．细菌

C．汗水

D．雨水

It can be inferred（推断） from the text that _________．

A．a synthetic ball takes in water faster than a leather ball

B．a leather ball is more difficult to make than a synthetic one

C．a synthetic ball is more expensive than a leather ball

D．a leather ball may increase its weight during a game