甲肝病毒感染早期诊断的主要血清学指标为（）。
参考答案：甲肝病毒IgM抗体
As the old empires were broken up and new states were formed, new official tongues began to _____ at an increasing rate.
A．bring up
B．build up
C．spring up
D．strike up
冷铁的作用（）
A.消除铸件局部热节
B.控制铸件的定向凝固或同时凝固
C.补缩
