diff --git a/20240918/csp2019pre/explain.md b/20240918/csp2019pre/explain.md
index 2dce873..0e9a4fc 100644
--- a/20240918/csp2019pre/explain.md
+++ b/20240918/csp2019pre/explain.md
@@ -10,7 +10,7 @@
 
 ### 2.C
 
-### 3.A
+### 3.D
 
 ```
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