The science teacher has to select 2 out of 5 excellent scientific papers to participate in the district competition. How many selection methods are there?
科学老师要从5篇优秀科技论文中选出2篇参加区比赛，一共有多少种选法．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">2 * 1</gadget>
<output>2</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<result>10</result>