A researcher attempted to clone two genes (X and Y) independently in a plasmid vector for over expression and purification in E. coli. All attempts to clone gene X were unsuccessful whereas gene 'Y' could be cloned easily. When the researcher attempted to clone gene 'X' in the plasmid clone containing gene 'Y', gene 'X' could be cloned. The following statements were proposed to explain the above results.

A. Protein encoded by gene 'Y' is not lethal to the cell.

B. Gene 'X' has introns, which prevents its expression in E. coli

C. Expression of 'X' protein is lethal to the cell.

D. The 'Y' gene product inhibits the activity of X' protein

Which one of the following options represents a combination of correct statements to explain the observations?