QT450-10材质铸件比HT250材质铸件强度高的主要原因是（）。
A.珠光体含量高
B.碳当量高
C.铁素体含量高
D.石墨呈球状
参考答案：D
Areas where students have particular difficulty have been treated ________ particular care.
A．by
B．with
C．as
D．in
Tariffs may ______ on commodities leaving an area or on merchandise entering an are
A．A. levyB. be leviedC. effectD. be effected
