a.

1. 基本的
2. 初级的，基础的
   He is a self-educated man. He didn't finish even elementary school.

   他是一个自学成材的人。他连小学也未读完。

3. 【化】元素的
   Tin is an elementary substance.

   锡是一种元素物质。

4. 自然力的
5. （问题）容易回答的

adj.

1. relating to the most rudimentary aspects of a subject; introductory.
   ▸simple.
2. Chemistry  not decomposable into elements or other primary constituents.