What is the function of the amp^r gene in a plasmid vector?

A)It provides several unique restriction enzyme recognition sites for opening up the plasmid.
B)It allows a researcher to separate bacterial cells with a plasmid from those without a plasmid.
C)It contains an origin for replication of the bacterial chromosome.
D)It allows a researcher to distinguish plasmids with an insert from those without an insert.
E)It is required for packing plasmid DNA into bacteriophage heads.