Which Methodology Was Used by Researchers to Identify the Nuclear

Which methodology was used by researchers to identify the nuclear localization sequence?

A) infection of cells in culture with viruses containing sequential deletions of amino acids in a protein known to localize to the nucleus, followed by microscopic determination of cellular localization
B) treatment of cells in culture with pharmacological agents that inhibit the nuclear pore complex, followed by microscopic determination of cellular localization
C) transfection of cells in culture with mutant human proteins containing sequences believed to be required for nuclear localization followed by microscopic determination of cellular localization
D) fluorescently labeling cells with known nuclear proteins and tracking changes in localization in the presence or absence of nuclear pore complex proteins
E) treatment of cells in culture with pharmacological agents that binds to the nuclear localization sequence and prevents nuclear transport, followed by microscopic determination of cellular localization