. Over the years I have collected five ________ foreign stamps, _________ of them are from the USA.

A. hundred;  two hundreds        B. hundred; two hundred

C. hundreds of; two hundred            D. hundreds; two hundreds

烟气含氧量变送器：应定期对氧化锆传感器进行清灰，检查安装连接处应无（）。应减少停电操作次数，以保证氧化锆传感器的使用寿命。