Find out the appropriate word in 1.

In the following passage there are blanks, each of which has been numbered. These numbers are printed below the passage and against each five words are suggested, one of which fits the blank appropriately. The structure of the delta is much ______1_______ that of the Bay of Bengal, but many have dubbed the formation of the delta a fluke of Nature. About two million years___2____, the original route of the Okavango River ended in a lake that is now the salt pans of the Makgadikgadi. Then about 50,000 years ago, a monumental earthquake in Southern Africa _____3______ a seismic flow disruption for the river, forming the 15,000 square-kilometre wetland ___4___ 11 trillion litres of water. So geologically speaking, the delta is quite young, but has also ______5_______ lifetimes of species thrive and wither.