一阶传感器系统阶跃响应的响应时间的长短由___________决定。

gy starting in China. Several people connect over the Internet and agree to contact a seller of a specific product so that they can buy the same item as a group at a lower price. And the sellers benefit by selling many products at once.When people agree on group buying,individual members of the buying group can vouch for (替 ……… 担 保 )a particular seller’s quality to the rest of the group,therefore Tuangou helps of similar phenomena in Euroupe and North America.However,most of the group buying in these places is organized and done not by the team members themselves,but through online go-betweens. go-betweens.
56. Where did Tudngou begin？
A.In America
B.In Europe
C.In North America
D.In China
Buyers purchase goods as a group in order to_________A.save money
B.connect over the Internet
C.buy different items
D.make friends
The sentence “ therefore Tuangou helps change a traditional distust of goods purchased from unknown sellers” implies that_________A.buyers used to doubt the quality of goods from unknown sellers
B.team buying is a good Chinese tradition
C.team buying is not accepted by people
D.buyers never purchase goods from unknown sellers
Sellers in the group buying benefit by_________A.paying less
B.selling many produdcts at once
C.bargaining with each buyer
D.selling goods at a higher price
What does the word "go-betweens" mean in the last paragraph？A.Online shopping
B.Going from one place to another
C.Traveling between two places
D.Persons or companies that pass messages between sellers and buyers

A、只要考虑视力和双眼视问题即可
B、应兼顾视力、双眼视和不等像这三方面的因素
C、有双眼视的，应尽量全矫正
D、成人矫正要考虑舒适性问题