已知︳a|=5，︳b|=7，且︳a+b|=a+b，则a﹣b的值为（ ）．
﹣2或﹣12
There is a wooden bridge ______the river _______ my house.
A．above, in front of
B．over, in the front of
C．above, in the front of
D．over, in front of
关于化学键的下列叙述正确的是[ ]
A．离子化合物中只含离子键
B．共价化合物中不含离子键
C．离子化合物中不可能含共价键
D．共价化合物中可能含离子键
