Read the following passage and answer the two items that follow. Your answers to these questions should be based on the passage only.
Crude mineral oil comes out of the earth as a thick brown or black liquid with a strong smell. It is a complex mixture of many different substances, each with its own individual qualities. Most of them are combinations of hydrogen and carbon in varying proportions. Such hydrocarbons are also found in other forms such as Bitumen, Asphalt and natural gas. Mineral oil originates from the carcasses of tiny animals and plants that live in the sea. Over millions of years, these dead creatures form large deposits under the sea bed; and ocean currents cover them with a blanket of sand and silt. As this mineral hardens, it becomes sedimentary rock and effectively shut out the oxygen, so preventing the complete decomposition of the marine deposits underneath. The layers of sedimentary rock become thicker and heavier. Their pressure produces heat, which transforms the tiny carcasses in to crude oil in a process that is still going on today.
Mineral oil deposits under the sea do not get completely decomposed because they:
The passage explicitly states that as the layers of sand and silt covering the dead marine organisms harden, they become sedimentary rock. This rock formation "effectively shut out the oxygen, so preventing the complete decomposition of the marine deposits underneath." Therefore, the exclusion of oxygen by the overlying rock layers is the direct reason for the incomplete decomposition of the mineral oil deposits.
The passage mentions that ocean currents cover the dead creatures with sand and silt. However, it does not attribute the prevention of decomposition to the constant washing by these currents. The critical factor identified is the subsequent hardening of these layers into sedimentary rock.
The passage describes crude oil as a complex mixture of hydrogen and carbon. This refers to the chemical composition of the resulting crude oil, not the mechanism that prevents the complete decomposition of the original organic deposits.
While the organisms live in the sea, the passage does not identify saline conditions as the reason for the incomplete decomposition of their carcasses. The primary reason provided is the exclusion of oxygen due to sedimentary rock formation.