球铁管标准工作长度偏差为（）mm。
A.±30、
B.±250、
C.+250、
D.+30
参考答案：B
光变色防伪油墨中含有MgF2，MgF2中F的化合价为 [ ]
A．＋1
B．＋2
C．－1
D．－2
______ Kitty ______ Cathy is going with you because one of them must stay at home.
A．Not only; but also
B．Neither; nor
C．Both; and
D．Either; or
