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