题目

完形填空。从A、B、C、D四个选项中，选出可以填入空白处的最佳答案。There is a good news for children in the countryside . We may still remember the girl  1 big eyes . Her big eyes are  2  us her dream : I wish to   3  ! In China, there are still   4  girls and boys like her . They want to go to school , but their  5  are too poor . If the family has two or three children , it is harder to  6   the money for all the children , so the parents often ask  7   to stay at home , and boys to go to school.Now they needn’t   8 the money . From 2006 on , c17．如图，固定的直杆ABC与水平地面成37°角，AB段粗糙，BC段光滑，AB长度l1=2m，BC段的长度l2=0.75m．质量m=1kg的小环套在直杆上，在与直杆成α角的恒力F作用下，从杆的底端由静止开始运动，当小环到达B时撤去F，此后小环飞离直杆，落地时的动能Ek=20J．（不计空气阻力，sin37°=0.6，cos37°=0.8，g取10m/s2）（1）求小环运动到B时的速度大小vB；（2）若夹角α=0°，小环在杆AB段运动时，受到恒定的阻力f=4N，其他条件不变，求F的大小；（3）若夹角α=37°，小环在AB段运动时，受到阻力f的大小与直杆对小环的弹力FN的大小成正比，即f=kFN，其中k=0.5，且小环飞离轨道后，落地时的动能不大于20J，求F的取值范围．