What DTM Event type and CSS selector should be applied?

A developer needs an Adobe Dynamic Tag Management (DTM) Event rule so that a custom event can be triggered when the dropdown/select field below has a change in its value:

<select names= "cat" id="cat" class="postform"></select>

What DTM Event type and CSS selector should be applied?
A . Event type of "dropdown" and a CSS selector of ".cat".
B . Event type of "focus" and a CSS selector of ".cat"
C . Event type of "change" and a CSS selector of "#cat"
D . Event type of "select" and a CSS selector of "#cat"

Answer: B