现金作为现代信用货币的主要形式之一，是( )。

A．中央银行的负债
B．中央银行的资产
C．商业银行的负债
D．商业银行的资产

If you want to get good grades, ____________, you should listen to the teacher _________ in class.

A．at first; careful

B．first of all; carefully

C．for the first time; careful

D．first of; carefully