实行7x24小时运行的支付系统有（）。
A．大额支付系统
B．小额支付系统
C．支票影像交换系统
D．同城清算系统
参考答案：B, C
A government spokesman represents the local government’s attitude so he has to ____ his words before responding to reporters.
A．pass
B．express
C．cover
D．weigh
简述锡石的形态。
