<span>The first enzyme is reverse transcriptase. The concept of
reverse transcription was much unknown at first. A reverse transcriptase is an enzyme used to
generate complementary DNA (cDNA) from an RNA template, a process termed
reverse transcription. It is mainly associated with retroviruses. As it
contradicted the central dogma of molecular biology, which states that DNA is
convert into RNA, which is then translated into proteins.</span>
D has a positive charge and located in nucleus
B is the answer to your question
<span>ileocecal valve or sphincter</span>