Direction: In the following passage there are blanks, each of which has been numbered. These members are printed below the passage and against each, five words are suggested, one of which fits the blank appropriately. Find out the appropriate words.

In geology, a trough is a linear structural depression that extends laterally over a distance. Although it is less steep than a trench, a trough can be a narrow basin or a geologic rift. These features often form at the rim of tectonic plates. The biggest  ______ (1) ______canyon is over 350 km long. It’s called ‘Foundation Trough’. Other canyons are not as long, but they are ______ (2) ______ too. These canyons were found with the help of radars after scanning the wide expanse of the frozen continent. ______ (3) ______ are incredibly important because they help control the flow of the ice. There are various oceanic troughs on the ocean floors.