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。
