Need help with a practice question

Which of the following would raise an error if the empID attribute's data type were defined as type ID in the DTD?
a. <name empID="17563">Joe Smith</name>
b. <name empID="ft:17563">Joe Smith</name>
c. <name empID="a17563">Joe Smith</name>
d. <name empID="">Joe Smith</name> Multiple
Select - Please select all of the correct answers (this question has 2 correct choices). Ans: a, d. - please explain.

Attributes that are declared to be of type ID must be legal XML name tokens.
Please check out the following discussion about the same topic:
http://www.coderanch.com/t/146982/po/certification/Type-ID-DTD

If the empID attribute's data type were defined as type ID in the DTD, the first character of this attribute may only be a letter, underscore or a colon.

Thank you for the prompted response. I got it now.

but i test all the four choices in the xmlspy,i found the b is also wrong,prefix "ft" is not allowed,and the string behind the prefix is not start with a letter!

Daniel,
<name empID="ft:17563">Joe Smith</name>
ft:17563 has nothing to do with prefix, namespace or some such.
ft:17563 is the content of the text node of the attribute.
All characters of "ft:17563" are allowed to qualify as legal xml name tokens.
[ March 18, 2003: Message edited by: Axel Janssen ]

to Axel:
the ":" in the string is not allowed,i test in xmlspy,she said:invalid value.when i del the ":",it's ok!

For the exam purpose, you should consider the XML recommendation as the absolute authoritative reference and not implementing tools like XMLSpy.
In the recommendation, it is stated in 3.3.1 Attribute Types that "Values of type ID must match the Name production." and the Name (and NameChar) productions are specified as
Name ::= (Letter | '_' | ':') (NameChar)*
NameChar ::= Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender
As you can see, the : character is perfectly admissible in attribute values. XMLSpy must have skipped that part

[ March 18, 2003: Message edited by: Valentin Crettaz ]

really thank you very much.&bow