Show Different Option Using Json?

I want to write a javascript that will show 'choice1' if 'abc' is selected,
and show 'choice2' if 'def' selected ...if 骃' selected it will show nothing .
How I can do that .

<SELECT name="cr_a">
<OPTION selected>Select One</OPTION>
<OPTION value=555>555</OPTION>
<OPTION value=abc>abc</OPTION>
<OPTION value=def>def</OPTION>
</SELECT>

---------------choice 1
<SELECT name="cr_abc_a">
<OPTION selected>Select One</OPTION>
<OPTION value=123>123</OPTION>
<OPTION value=456>456</OPTION>
</SELECT>
----------------choice2
<SELECT name="cr_def_a">
<OPTION selected>Select One</OPTION>
<OPTION value=ddd>ddd</OPTION>
<OPTION value=eee>eee</OPTION>
</SELECT>