逆变式弧焊电源比一般电源工作频率提高了()倍。
A.10~20
B.20~50
C.50~100
D.100~600
参考答案:D
Europeans always hold their fork in the left hand, ________Americans switch it between their left and right hands.
A.before
B.because
C.while
D.since
简述商业银行贷款五级分类的定义及主要特征。