Q.161 Zoology ⭐ 360° Option Elimination Active

NEET 2026 Zoology Question 161 Solution

Topic: Organisms and Populations · Official NTA Booklet Code 50

Question

Consider a population of 10 million cells. Given the per-capita birth rate of 0.002 (per unit time) and the per-capita death rate of 0.002 (per unit time), the expected number of cells after 10 generations is.

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Verified Against NTA Final Answer Key

Option (3) : 10 million

📚 Organisms and Populations Easy

📋 Given Parameters & Problem Setup

Initial Population (N₀) 10 million

Per Capita Birth Rate (b) 0.002

Per Capita Death Rate (d) 0.002

Time Span 10 generations (no migration)

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The 30-Second Exam Hall Shortcut: ⚡ Zero Population Growth (ZPG) Rule

⏱️ Save ~40 seconds

b = d ⟹ r = 0 ⟹ Population Size NEVER CHANGES!

1 Calculate intrinsic rate of natural increase: r = b - d = 0.002 - 0.002 = 0.

2 Population growth equation: dN/dt = rN = 0 × N = 0.

3 Since rate of change is zero, population size at any generation t is: N(t) = N₀ = 10 million.

4 No exponential explosion, no extinction.

5 Pick Option (3) = 10 million in 5 seconds flat!

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Why You Can 100% Trust This Shortcut: NCERT Biology · Organisms and Populations (Population Growth Models: Logistic growth equation dN/dt = rN(K-N)/K).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Intrinsic Rate of Natural Increase (r)

In population ecology, the intrinsic rate of natural increase (r) is determined by subtracting the per capita death rate (d) from the per capita birth rate (b).

r = b - d = 0.002 - 0.002 = 0

• r is the vital statistic measuring inherent capacity of a population to grow.

Step 2

Apply Exponential Growth Equation

The population size after t time periods is given by the integral form of exponential growth. When r = 0, e^(rt) = e^0 = 1, meaning the population remains completely stationary.

N_t = N_0 e^{rt} = 10 \text{ million} \times e^{0 \times 10} = 10 \text{ million} \times 1 = 10 \text{ million}

• Stationary population exhibiting Zero Population Growth (ZPG).

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Population would only double if r > 0.

⚠️

Student Fallacy (Why candidates pick this trap): Arbitrarily doubling the population over 10 generations.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Reduction would require death rate to exceed birth rate (d > b).

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Student Fallacy (Why candidates pick this trap): Assuming natural decline despite equal birth and death rates.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% mathematically and ecologically exact. Since b = d, r = 0, and population remains constant at 10 million.

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Why this is airtight: None; straightforward application of r = b - d.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. The population is not going extinct.

⚠️

Student Fallacy (Why candidates pick this trap): Equating r = 0 with population = 0.

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NTA Trap Alert: Growth Rate Zero vs. Population Zero Confusion

Careless students sometimes confuse the rate of growth (dN/dt = 0) with the population size (N = 0). A growth rate of zero means size remains CONSTANT.

🛡️ How to avoid the -1 mark: Remember: dN/dt = 0 means ΔN = 0, so N_final = N_initial.

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Core NEET Syllabus Standard

NEET Biology Core Syllabus · Organisms and Populations Standard Benchmark

Core Scientific Principle Tested:

"dN/dt = (b - d) × N; when b = d, dN/dt = 0 and population size remains constant."

Syllabus Focus: Population Growth Models

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Births Balance Deaths = Baseline Stays (10M -> 10M)!

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NEET 2026 Zoology Question 161 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Zero Population Growth (ZPG) Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Intrinsic Rate of Natural Increase (r)

Apply Exponential Growth Equation

360° Option Elimination Engine Key Feature

NTA Trap Alert: Growth Rate Zero vs. Population Zero Confusion

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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