— You________have finished your composition.

— I had meant to, but before I could, the bell rang and  we had to hand in our test papers. [      ]

A. were supposed to  

B. were able to

C. had to 

D. were determined to

某高速公路管理部门，采用射频识别技术开发了电子不停车收费(ETC)系统在各收费站或收费龙门架上安装了 ETC 系统识读器。当装有射频标签的汽车经过时，识读器将自动识别并刷新射频标签内的备付金金额，再通过 ETC 系统的计算机网络系统迅速将车牌号、车型、备付金金额等数据上传给相关收费银行和收费结算中心，由银行根据收费标准扣减该车对应的备付金，并相应地将该部分款项划拨到收费结算中心。结算中心再根据车辆行驶路段将所收款项分拆给不同的路桥收费公司。如果该车辆的备付金剩余额低于警示金额，射频标签将发出警示，敦促车主到银行存储各付金。该系统也可满足车主和政府管理部门的终端和电话查询服务。

射颇识别系统由哪几部分构成(3 分)