若△ABC的三边长a，b，c满足a2+b2+c2=ab+ac+bc，则△ABC为()．
A. 等腰三角形
B. 直角三角形
C. 等边三角形
D. 等腰直角三角形
E. 以上答案均不正确
查看答案
在图示三个振动系统中，物块的质量均为 m，弹簧的刚性系数均为k，摩擦和弹簧的质量不计。设图(a)、(b)、(c)中弹簧的振动频率分别为f1、f2、f3，则三者的关系为()。
A. f1=f2≠f3
B. f1≠f2=f3
C. f1=f2=f3
D. f1≠f2≠f3
In addition, Froebel was concerned that the spread of industrialization would negatively affect the family, but he believed that______.
A. kindergartens could increase the social ability of mothers and children
B. kindergartens were associated with liberalism and free-thinking
C. they would not be banned by the Prussian government in the future
D. kindergartens could increase the social position of mothers and children
微信支付 
支付宝支付