Module S – Interactive Quiz – 20 Multiple-Choice Questions
Click an answer to check. Your score is tracked below.
0 / 20 answered
Q1Understand
What does “fork resolution” mean in Bitcoin?
Explanation
When two miners find valid blocks simultaneously, the network converges on the chain with the most cumulative work.
Q2Understand
What is an “orphaned” (stale) block in Bitcoin?
Explanation
Orphaned blocks are valid but discarded when another chain accumulates more cumulative work.
Q3Understand
What does Bitcoin’s “cumulative work” measure?
Explanation
Cumulative work sums the expected hashes required for every block; it is the true chain selection criterion.
Q4Understand
How often do natural forks typically occur on the Bitcoin network?
Explanation
Propagation delays occasionally cause two valid blocks to be found near-simultaneously, creating brief forks.
Q5Apply
A miner discovers a valid block but it takes 12 seconds to propagate. Another miner also finds a valid block during that window. What is the immediate result?
Explanation
Both blocks are locally valid; nodes hold whichever they saw first until one chain extends and wins.
Q6Apply
An online retailer sells a $200 item and wants less than 0.1% reversal risk against a 10% attacker. How many confirmations are needed?
Explanation
Nakamoto’s formula shows 3 confirmations reduces reversal probability below 0.1% against a 10% attacker.
Q7Apply
A node receives Block X from Miner A. Ten seconds later, competing Block Y arrives at the same height. What does the node do?
Explanation
Nodes apply the first-seen rule locally but switch if a competing chain grows longer (more cumulative work).
Q8Apply
Chain A: 10 blocks, difficulty 100 each. Chain B: 8 blocks, difficulty 150 each. Which chain does Bitcoin select?
Explanation
Bitcoin selects by cumulative work, not block count: 8 × 150 = 1,200 > 10 × 100 = 1,000.
Q9Apply
After a fork resolves, a transaction from the orphaned block does NOT appear in the winning chain. What is the most likely explanation?
Explanation
Orphaned-block transactions return to the mempool, but a double-spend in the winning chain prevents re-inclusion.
Q10Apply
Bitcoin’s difficulty adjusts every 2,016 blocks. If miners double hash power, what happens to cumulative work per block after adjustment?
Explanation
Difficulty rises to keep block time at 10 minutes; cumulative work per block is unchanged at target.
Q11Apply
A Bitcoin ATM processes $50 purchases with 0 confirmations. A customer attempts a double-spend. Why is this economically irrational?
Explanation
Mining hardware, electricity, and risk of detection far outweigh a $50 gain; the incentives deter attack.
Q12Apply
Compact Blocks (BIP 152) reduces propagation time from seconds to milliseconds. How does this affect fork rates?
Explanation
Faster block relay shrinks the window in which two valid blocks can coexist, reducing fork probability.
Q13Analyze
Bitcoin’s whitepaper says “longest chain” but the code implements “most cumulative work.” When do they differ?
Explanation
Block count and cumulative work agree only when all blocks share identical difficulty; variable difficulty breaks this equivalence.
Q14Analyze
GHash.io briefly exceeded 50% of Bitcoin’s hash power in 2014 but did not attack the network. Why?
Explanation
A successful 51% attack destroys confidence in Bitcoin, collapsing the very asset the attacker mines and holds.
Q15Analyze
The 2010 Value Overflow bug created 184 billion BTC. The community released a fix and built a competing chain. What type of consensus resolved this?
Explanation
Nodes chose to upgrade and follow the patched chain, demonstrating that social consensus underlies the technical layer.
Q16Analyze
A blockchain with 2-second blocks has a 15% fork rate vs Bitcoin’s 0.3%. What causes the difference?
Explanation
If propagation takes 1 second and block time is 2 seconds, a 50% overlap chance exists vs. <1% at 10 minutes.
Q17Analyze
Why do exchanges require different confirmation counts (e.g., Coinbase 3, Kraken 4, Binance 1)?
Explanation
Confirmation thresholds are business decisions: higher-value deposits or lower risk tolerance require more confirmations.
Q18Analyze
In Nakamoto’s formula, what happens as attacker hash power q approaches 50%?
Explanation
Near 50%, the attacker can almost keep pace with the honest chain; exponentially more confirmations are needed.
Q19Evaluate
Bitcoin’s $15B/year mining cost is called “wasteful.” What is the strongest economic counter-argument?
Explanation
Security spend should be evaluated relative to the value secured; $15B protects a $1.8T asset with no trusted third party.
Q20Evaluate
A proposal replaces “most cumulative work” with “most cumulative coin-days-destroyed.” What is the strongest objection?
Explanation
Physical energy cost is exogenous and unfakeable; coin-days-destroyed can be gamed by wealthy insiders at low marginal cost.