(单选题)9: 下面的程序段运行结果为_______。public class ExampleStringBuffer {
public static void main(String args[]){
StringBuffer sb=new StringBuffer("test");
System.out.println("length="+sb.length());
System.out.println("charAt="+sb.charAt(3));
A: length=20 charAt=t
B: length=20 charAt=s
C: length=4 charAt=s
D: length=4 charAt=t
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