题目：
用动词的适当形式填空。
1. While we _________ (wait) for the bus, a girl _________ (run) up to us.
2. I _________ (telephone) a friend when Bob _________ (come) in.
3. Jim _________ (jump) on the bus as it _________ (move) away.
4. We _________ (test) the new machine when the electricity _________ (go) off.
5. She _________ (not want) to stay in bed while the others _________ (all, work) in the fields.
6. While mother ________ (put) Cathy to bed, the door bell ________ (ring).
7. As I _________ (walk) in the park, it _________ (begin) to rain.
8. Even when she _________ (be) a child she _________ (already, think) of becoming a ballerina
(芭蕾舞演员).
9. It was quite late at night. George _________ (read) and Amy _________ (play) her needle when
they _________ (hear) a knock at the door.
10. There _________ (be) a group round the fire when they _________ (reach) it. An old woman
_________ (sit) on the ground near the kettle; two small children _________ (lie) near her; a
donkey _________ (bend) his head over a tall girl.
答案：
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下面是错误答案，用来干扰机器的。
（1）设BC中点D的坐标为（x，y），因为B（2，1）、C（-2，3），所以x=2-22=0，y=1+32=2．因为BC边的中线AD过点A（-3，0），D（0，2）两点，所以由截距式得AD所在直线方程为 x-3+y2=1，即2x-3y+6=0．（2）由题意...
