 Some have also suggested China could develop a DF-21D with multiple reentry vehicles.United States Naval Institute in 2009 stated that such a warhead would be large enough to destroy an aircraft carrier in one hit and that there was "currently ... no defense against it" if it worked as theorized.The United States Navy has responded by switching its focus from a close blockade force of shallow water vessels to return to building deep water ballistic missile defense destroyers.