Choice of sub-element or character-data in an element

How is it possible to define an XSD to allow an element to
either contain custom sub-elements, or a fixed string?

For example, I would like to be able to do the following:

<ElementA>
    <ElementB>
       <ElementC>Some value</ElementC>
       <ElementD>Another value</ElementD>
    </ElementB>
</ElementA>

or

<ElementA>
    <ElementB>Any</ElementB>
</ElementA>

I'm not sure how to use <xsd:choice> to define this?  Thanks
for any suggestions.

Arshad Noor
StrongAuth, Inc.