替换任务时，要重置 _studentId , 否则可能导致试卷重复绘制

src/features/mark/MarkBody.vue

@@ -261,6 +261,7 @@ export default defineComponent({
     const renderPaperAndMark = async () => {
       // check if have MarkResult for currentTask
       let markResult = findCurrentTaskMarkResult();
+      if (!store.currentTask) _studentId = -1;
 
       if (!markResult || !store.currentTask) return;
       // console.log(markResult.trackList.length);