直流输电的潮流反转分为哪两种类型（）
A.正常潮流反转
B.降压潮流反转
C.轻载潮流反转
D.紧急潮流反转
参考答案：A, D
精益生产的两大支柱是JIT和TPM。（）
When I returned home from a long journey, I found my house______.
A．had been broken in B．was broken into C．had broken in D．had broken into
