In Terms of Internal Validity, Which of the Following Is

In terms of internal validity, which of the following is NOT a problem with matching?

A) You can't find participants who are identical in every respect.
B) You can't measure and match subjects on every single variable.
C) You probably won't know every single relevant variable that subjects should be matched on.
D) You have to throw out subjects who don't match up with any of your other subjects, resulting in mortality.