Read the passage on crude mineral oil and answer the question: According to the passage, the time taken for the marine deposits to harden into rocks is how long?

Introduction / Context:
This reading comprehension question focuses on the timescale involved in the formation of sedimentary rocks and, ultimately, crude oil. The passage emphasises that geological processes take extremely long periods, and the exam question checks whether you have understood and remembered this detail accurately. Understanding such information is useful not only for exams but also for general scientific awareness.

Given Data / Assumptions:

• The passage mentions: "Over million of years, these dead creatures form large deposits under seabed and ocean currents cover them with a blanket of sand and silt."
• The context suggests that the transformation into sedimentary rock and then crude oil occurs over very long geological times.
• Options offer different time spans: decades, centuries, thousands of years, and millions of years.
• We assume the reader has noted the phrase "million of years" in the text.

Concept / Approach:
Geological changes, such as the formation of sedimentary rock and fossil fuels, do not happen quickly. They generally take millions of years. The passage uses this standard scientific idea, and the correct answer must reflect this very long timeframe. Exam setters include shorter timespans as distractors for candidates who do not pay attention to exact wording or are unfamiliar with geological processes.

Step-by-Step Solution:
Step 1: Locate the relevant sentence: It talks about dead creatures forming deposits "over million of years" before being covered and turned into rock. Step 2: Interpret "over million of years" as a very long timeframe, clearly beyond thousands of years. Step 3: Examine option B: "millions of years." This exactly matches the idea from the passage. Step 4: Examine option A: "a few centuries." A century is only 100 years, and a few centuries would be too short for large scale geological changes of this kind. Step 5: Examine option C: "a few decades." A decade is only 10 years, which is far too short. Step 6: Examine option D: "thousands of years." Although much longer, it is still significantly shorter than "millions of years" and does not match the passage exactly.

Verification / Alternative check:
You can verify by considering general knowledge from science: fossil fuels like coal, petroleum, and natural gas are known to take millions of years to form. Textbooks repeatedly emphasise this point. The passage is consistent with that scientific fact. Therefore, any option suggesting decades, centuries, or even just thousands of years cannot be correct. The phrase "millions of years" is the most precise match for the passage and for accepted scientific understanding.

Why Other Options Are Wrong:

• a few centuries: Represents only hundreds of years, which is far too short for sedimentary rock formation and oil deposits.
• a few decades: Represents tens of years, which is clearly inadequate for slow geological processes.
• thousands of years: While longer, it still does not match the explicit phrase "million of years" used in the passage, and thus must be rejected.

Common Pitfalls:
Some candidates skim the passage and remember only that "a very long time" is involved, then choose "thousands of years" believing it to be long enough. Others may misread "million" as "thousands". To avoid these errors, always pay attention to numerical data and units in the passage. Underlining numbers, dates, and quantities while reading is a helpful strategy for tackling detail based questions like this one.

Final Answer:
According to the passage, the process takes millions of years for the marine deposits to harden into rocks.