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