Update app.py

app.py

@@ -64,7 +64,7 @@ SUBJECT_SUBTOPICS = {
         "optimization",
         "stokes theorem",
         "green's theorem",
-        "line intergals",
+        "line integrals",
         "parametric equations",
         "spherical coordinates"
     ],
@@ -824,7 +824,7 @@ def generate_question(subject, difficulty, question_type, subtopic, use_enhancem
         
         request_history.append(now)
         
-        selected_topic = subtopic
+        selected_topic = subtopic if subtopic else random.choice(SUBJECT_SUBTOPICS[subject])
         logger.debug(f"Using selected subtopic: {selected_topic}")
         
         difficulty_params = get_difficulty_parameters(difficulty)