保险备用信用证
参考答案：
是指开证行保证申请人的某一保险或再保险义务的备用信用证。
There ________ quite a few monkeys in the mountains. [ ]
A. is used
B. used to be
C. used to are
D. are used to be
试叙述PLC输入／输出模块的性能及技术要求有哪些？
