$.$ The thymidine analog $3\text{'}-$azidothymidine $($AZT$)$ blocks replication of human immunodeficiency virus $($HIV$).$ AZT is converted in cells to the $5\text{'}-$triphosphate nucleotide derivative and then incorporatioed into cDNA copies of the infecting HIV RNA by the HIV's own reverse transcriptase. The cDNA terminates prematurely at the point of the inserted analog. The most likely reason that AZT is not equally inhibitory toward replication of human DNA is that the $5\text{'}-$triphosphate nucleotide derivative is $\_\_\_\_\_\_\_\_\_\_\_.$ A$.$ capable of base$-$pairing with the riboadenosine in the HIV RNA, but not with the deoxyadenosine in the human DNA B$.$ replace with the normal deoxynucleotide during DNA replication C$.$ unable to enter the nucleus, where DNA is replicated D$.$ hydrolyzed by nuclear enzymes to the thymidine$-5\text{'}-$triphosphate, so the azido$-$form is not incorporated into replicating DNA E$.$ bound with much lower affinity $($resulting from a higher Km$)$ to human DNA polymerase than to HIV reverse transcriptase.