一根轻质弹簧一端固定，用大小为F1的力压弹簧的另一端，平衡时长度为l1；改用大小为F2的力拉弹簧，平衡时长度为l2．弹簧的拉伸或压缩均在弹性限度内，该弹簧的劲度系数为（ ）
He Jing______to give a speech at the graduation ceremony at Harvard University last month．（ ）
invites
invited
is invited
was invited
﹣It is reported that cancer can be controlled at a temperature lower than 100℃below zero．﹣Really？______ nice surprise!（ ）
How
What
What a
What an
Unless the weather______，we'll have to cancel the picnic．（ ）
improve
improves
improved
will improve
School violence______much attention of the whole society and people are calling on the government to make laws against it as early as possible．（ ）
drew
will draw
has drawn
was drawing
According to a recent survey，______three fifths of working mothers in china don't want to have a second child．（ ）
mostly
especially
partly
nearly
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