单项选择题
西门子S7－200系列PLC内部由存储单元组成的编程元件类型很多，下列四种编程元件符号中属于“特殊标志位”类型的是()。
单项选择题 I can’t find the key __ the door. Maybe I lost it.
单项选择题 The young Thomas Edison __ an inventor.
单项选择题 _ came that our sports team had won 15 gold medals at the Olympics.
