A researcher inserted DNA fragments from an organism into expression vector plasmids and introduced the modified plasmids into bacterial cells. Which of the following methods would be an effective means of identifying which clones contain a specific gene of interest?

A) Sequence the DNA of the plasmids from each isolate.
B) Prepare and analyze restriction fragments of the plasmids.
C) Isolate clones that still have the vector's genetic marker.
D) Assay for activity of the gene product.
E) Use reverse transcriptase to prepare cDNA copies.