Modify header.php

Hello there
I now have several different CSS files
Different CSS files are needed for homepage, post, and page
I modified header.php
like this
<? php if (is_home ()) {?>
<link rel = “stylesheet” href = “cssA” type = “text / css” />
<? php}
elseif (is_single ()) {?>
<link rel = “stylesheet” href = “cssB” type = “text / css” />
<? php}
elseif (is_page ()) {?>
<link rel = “stylesheet” href = “cssD” type = “text / css” />
<? php}?>
But none of my code worked
Why is the modification of header.php invalid?
Thank you

anyone?