Module A – Interactive Quiz – 20 Multiple-Choice Questions
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Q1Understand
What is the “double-spending problem” in digital payments?
Explanation
Digital data can be copied, so without consensus, the same coin could be sent to multiple people.
Q2Understand
On the trust spectrum shown in the lecture, what does moving from left to right represent?
Explanation
Each step rightward removes a layer of human trust and replaces it with a verifiable rule.
Q3Understand
Which three words best summarize what a blockchain is?
Explanation
A blockchain is a shared notebook that everyone can read, nobody can erase, and no single person controls.
Q4Understand
What is the “avalanche effect” in hash functions?
Explanation
Changing “Hello” to “hello” produces a completely different SHA-256 fingerprint.
Q5Apply
Alice has 1 BTC and broadcasts two transactions: 1 BTC to Bob and 1 BTC to Carol. What does the network do?
Explanation
The network reaches consensus on which transaction is valid; the duplicate is rejected.
Q6Apply
Block 100 has fingerprint abc123. Block 101 stores “previous = abc123.” An attacker changes Block 100. What happens?
Explanation
Tampering changes the hash, breaking the chain link; all subsequent blocks detect the mismatch.
Q7Apply
In Proof of Work, a miner spends $50,000 on electricity and submits a valid block worth $300,000. What is the profit?
Explanation
Profit = reward ($300,000) minus electricity cost ($50,000) = $250,000.
Q8Apply
A blockchain prioritizes high throughput and strong security but uses only 20 validators. Which trilemma property is sacrificed?
Explanation
Fewer validators means less decentralization; this is how chains like Solana achieve high speed.
Q9Apply
Bitcoin processes about 7 transactions per second while Visa handles 65,000. According to the lecture, what explains this gap?
Explanation
Getting 50,000 computers to agree without a leader is far slower than one company processing payments.
Q10Apply
A company where only one department controls all data considers using blockchain. The decision tree says:
Explanation
The decision tree exits at “multiple parties with trust deficit”; one trusted party should use a database.
Q11Apply
The first real-world purchase using Bitcoin was 10,000 BTC for two pizzas in 2010. This event proved that:
Explanation
The pizza transaction demonstrated that digital coins enforced by consensus have real purchasing power.
Q12Apply
A cheating miner spends $50,000 on electricity and submits a fraudulent block. The network rejects it. What is the outcome?
Explanation
Cheating wastes the electricity cost with zero return; honesty is the profitable strategy.
Q13Analyze
Mt. Gox lost $450M in Bitcoin in 2014, yet the Bitcoin blockchain continued operating. What does this distinction reveal?
Explanation
The blockchain layer was secure; the centralized exchange’s own security was the weak point.
Q14Analyze
The lecture states that Bitcoin’s security budget exceeds $15 billion per year. Why does a larger budget make attacks harder?
Explanation
A 51% attack requires more computing power than the rest of the network combined, which costs more than any gain.
Q15Analyze
Terra/Luna collapsed in 2022, destroying $40 billion. The lecture says the blockchain “worked perfectly.” What actually failed?
Explanation
The blockchain executed code correctly; the flawed incentive design caused the collapse.
Q16Analyze
The lecture places cash, banks, PayPal, and blockchain on a trust spectrum. What pattern emerges as you move toward blockchain?
Explanation
The spectrum shows a progressive shift from institutional trust to cryptographic verification.
Q17Analyze
A Bitcoin block header is 80 bytes, but the block body can be up to 4 MB. Why is this separation important?
Explanation
The small header size enables mobile phones and lightweight devices to participate in verification.
Q18Analyze
China’s e-CNY uses blockchain technology but is centrally controlled. How does this contradict Bitcoin’s original design goal?
Explanation
Bitcoin aimed to remove central control; CBDCs use the same technology to strengthen it.
Q19Evaluate
Five international aid organizations want to share transparent donation records but do not trust each other. Evaluate whether blockchain is appropriate.
Explanation
The decision tree confirms: multiple parties + trust deficit + need for immutability = blockchain is appropriate.
Q20Evaluate
Bitcoin uses 150 TWh/year (comparable to Argentina), while Proof of Stake reduces energy by 99.95%. Should all blockchains switch to Proof of Stake?
Explanation
Each consensus mechanism involves different trade-offs; the right choice depends on the network’s priorities.