A E. coli cell of lacl+lacP+lacO+ lacZ-lacY+/lacl-lacP+lacOClacZ+lacY-will:
Always express lacZ, and express lacY only in the presence of lactose.
Always express lacy, and express lacZ only in the presence of lactose.
Only express lacY, only in the absence of lactose.
Always express lacZ and lacY.
Never express lacZ or lacY.