How can it be C? A it is.
X says that tests sometimes produce false results.
Author says that these fears are without basis.
Hence, author wants to provide support for the accuracy of those tests. A says that those tests are often times inaccurate and directly attacks the author's claim and weakens the argument. Whether the drug is accurate or not when drug was consumed in the 48 hours prior to testing is out of scope. We are only
interested in the accuracy of the test if drug was consumed in the last 48 hours