If you were to descend through the ocean in a submersible, what changes in the ocean water could you observe?

I would assume that the ocean would start getting colder, and darker because the deeper the ocean goes, the less the sunlight gets. If you go deep enough, the ocean will just turn pitch black.

When we travel down into the deeper part of the ocean, we observe the following things-

The water in the upper part of the ocean appears to be blue in color but deep down, its color varies becoming somewhat darker.There is no sunlight below a depth of about 200 m from the mean sea level.The salinity also changes as we go down. In the upper part the water contains high salinity upto about 100 m and from 100 m to 1000 m the salinity again decreases with depth. Further salinity increases beyond 1000 m.Thermocline layer can be observed/felt which shows that the temperature in the upper and the lower part of the ocean are different.Deep down the ocean, more complex organisms are found to exist which can survive easily under extreme pressure condition.

The answer is c) pyrite