问题
X线照片对比度的正确表达式为
A.K=D1+D2
B.K=D1-D2
C.K=D12+D22
D.K=D12-D22
E.K=(D1-D2)2
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
