下面代码的运行结果是( )。
public class ConcatTest
public static void main (String[ ] args)
String str1 = "abc";
String str2 = "ABC";
String str3 = str1. coneat(str2);
System. out. println(str3);

A．abc

B．ABC

C．abcABC

D．ABCabc

古人用天干、地支纪年，天干有12个，地支有10个。( )