练习打卡，麻烦老师检查～！谢谢～！

plan.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE teaching-plan SYSTEM "plan.dtd">
<teaching-plan>
	<course num="001">
		<course-name>College English</course-name>
		<class-hour>36</class-hour>
		<exam-form>Test</exam-form>
	</course>
	<course num="002">
		<course-name>Advanced Math</course-name>
		<class-hour>70</class-hour>
		<exam-form>Test</exam-form>
	</course>
	<course num="003">
		<course-name>Elementary Computing</course-name>
		<class-hour>108</class-hour>
		<exam-form>Computer-based Test</exam-form>
	</course>
</teaching-plan>

plan.dtd
​<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT teaching-plan (course+)>
<!ELEMENT course (course-name,class-hour,exam-form)>
<!ATTLIST course num CDATA "">
<!ELEMENT course-name (#PCDATA)>
<!ELEMENT class-hour (#PCDATA)>
<!ELEMENT exam-form (#PCDATA)>