In the following passage some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.

Among the largest species of cats in the world, tigers are powerful hunters with (1)________ teeth, strong jaws and agile bodies. They range across Asia from Russia all the way to Sumatra and mainland Southeast Asia. Researchers still have much to learn (2) ________ these beautiful, endangered (3) ________. There is currently one recognized species of tiger, Panthera tigris. Scientists have further (4) ________ the tiger into nine subspecies: the extinct Bali, Caspian and Javan subspecies, and the living Malayan, Sumatran, South China, Indochinese, Bengal and Amur (or Siberian) subspecies. Of these six subspecies, AZA-accredited zoos currently manage three: Amur, Malayan, and Sumatran. The Smithsonian's National Zoo and Conservation Biology Institute is home to Sumatran and Amur tigers; Sumatran tigers are (5) ________ as Critically Endangered on the International Union for Conservation of Nature's Red List, and Amur tigers have been classified as Endangered.