问题
3,6,8,13,20,（),51 A.31B.28C.42D.32
3, 6, 8, 13, 20, (), 51
A.31
B.28
C.42
D.32
请帮忙给出正确答案和分析，谢谢！
Fatal error: Uncaught [XSException] ../../../usr/local/xunsearch/sdk/php/lib/XS.php(2559): Connection refused(C#111, localhost:8384)
thrown in /usr/local/xunsearch/sdk/php/lib/XS.php on line 2559
