How to disable select option based on other select options?

How to disable select option based on other select options?

I Have a code like this
first select box
  1. <select id="1">
  2. <option value="a ">a</option>
  3. <option value="b ">b</option>
and other select box like this
  1. <select id="2">
  2. <option value="a ">a</option>
  3. <option value="b ">b</option>
Whenever user select an options from select id 1 just say it is 'a' then user should not able to select 'b' from the select box 2. 

I am having really a hard time using jquery. 

Any help would be appreciated. 
Thank you.