Option (d) is correct: On the surface of human cells is an enzyme called ACE2, which acts as the receptor that enables SARS- CoV2 to launch its attack. The virus’s spike protein binds to the receptor, then fuses with the cell surface, and releases its genetic material (RNA in the case of SARS- CoV2) into the cell. The coronavirus that causes SARS, called SARS-CoV, uses the same ACE2 receptor to invade a cell.