题目

He had wanted to write ____children's book for many years, but one thing or another always got in _________ way. A . a; / B . the; the C . /; the D . a; the 答案：D17．一根轻质弹簧一端固定，用大小为F1=10N的力压弹簧的另一端，平衡时长度为L1=8cm；改用大小为F2=15N的力拉弹簧，平衡时长度为L2=18cm，弹簧的拉伸和压缩都在弹性限度内，该弹簧的劲度系数为（　　）A．0.5N/mB．2.5N/mC．125N/mD．250N/m