单选题
The largest system serving e-mail messengers is the Internet, a______ of millions of computers linked worldwide.
【正确答案】
C
【答案解析】network原指一种“网状系统”，广播网，电视网等，这里指计算机网络：unity意为“团结，联合，统一，一致”； combination 意为“结合：联合：合并”，如：His character is a combination of strength and kindness．他的性格是刚与柔的结合。connection意为“连接，连结：联系”，如：The television isn't working，is there a loose connection? 电视机坏了，有没有哪一点接线不紧?