Passage: Modern warships are generally divided into seven main categories, which are: aircraft carriers, cruisers, destroyers, frigates, corvettes, submarines and amphibious assault ships. Battleships comprise an eighth category, but are not in current service with any navy in the world. Only the deactivated American Iowa-class battleships still exist as potential combatants, and battleships in general are unlikely to re-emerge as a ship class without redefinition. The destroyer is generally regarded as the dominant surface-combat vessel of most modern blue-water navies. However, the once distinct roles and appearances of cruisers, destroyers, frigates, and corvettes have blurred. Most vessels have come to be armed with a mix of anti-surface, anti-submarine and anti-aircraft weapons. Class designations no longer reliably indicate a displacement hierarchy, and the size of all vessel types has grown beyond the definitions used earlier in the 20th century. Another key differentiation between older and modern vessels is that all modern warships are "soft", without the thick armor and bulging anti-torpedo protection of World War II and older designs.

Question: How many categories are modern warships divided into?
seven