XPath for selecting element without an attribute

Hello,

I'm curious of whether it's possible to build an XPath able to select an
element without an attribute.

For example, consider the following:

<A>
  <B> One </B>
  <B C="true"> Two </B>
  <B C="false"> Three </B>
</A>

I'm hoping to find the B element without the attribute, but not any others.

I have been trying things like the following //A/B[@C=''], attempting to key
off an empty string, but haven't had any luck.  Also, I'm going against a
schema which I can't change.  I'm assuming that this is possible, right?

Thanks for any help,

Aaron