It's a bit more complicated than that. I actually had to look this up to double check it.

If the combination is over 26,000 AND trailer is over 10,000, you need a class A.

If the tow vehicle is over 26,000 but trailer is 10,000 or under, you need a class B

With a class C/D there is no restriction on trailer weight as long as the combination is not over 26,000. You can also exceed 26,000 combined weight with a class C/D as long as the trailer is not over 10,000 AND the tow vehicle itself is not over 26,000