Reporter Genes Such as Those for Green Fluorescent Protein (GFP)are

Reporter genes such as those for green fluorescent protein (GFP) are found on many commercial plasmids.What are these genes used for?

A) to determine if the plasmid is in the host
B) to determine if a gene is present in a library
C) to determine if a foreign protein is being expressed on a vector
D) to determine the relative strength of a promoter sequence