Prueba de codificación de solidez

Review the following Solidity code:

We want to add a fallback function to the contract. Pick the correct fallback function and correct statements:

E: Fallback function is executed if contract receives plain Ether without any data
F: Fallback function is executed if caller calls a function that is not available
G: If multiple unnamed functions are defined for a contract, the cheapest function is used as fallback function

Review the following Solidity code:

Pick the correct statements:

A: private increment function defined in contract_e is accessible in derived contract d_c but not accessible outside the derived contracts.
B: private increment function defined in contract_e is accessible only inside contract_e and not accessible to derived contracts as well.
C: internal variable str defined in contract_e is accessible in derived contract d_b but not outside the derived contracts.
D: internal variable str defined in contract_e is accessible only inside contract_e and not accessible to derived contracts as well.

A 51% attack is perhaps the most discussed threat to any public Blockchain asset. A 51% attack happens when a certain miner or mining pool, that owns more than 50% of all the network’s hashing power, acts maliciously to disturb the network’s operation.

For example: Two Bitcoin Cash mining pools, carried out a 51% attack on the Bitcoin Cash blockchain in order to stop an unknown miner from taking coins that they weren’t supposed to have access to, while the network forked.

Which of the following CAN be done by a miner employing a 51% attack?

A: Double spend their coins
B: Reverse confirmed transactions
C: Steal funds from a certain address
D: Prevent transactions from being confirmed
E: Create false transactions that never happened

Breed Chasing, solo engineer is planning to create new cryptocurrency, NEWCRAIGLIST and build a blockchain that facilitates buying and selling second-hand items with NEWCRAIGLIST from each other. Breed expects the platform to be used for international transactions and devised the following process:

- Before a transaction is considered complete, the purchaser must put the funds in an escrow service
- The sender must enter the courier tracking number into that same escrow

Palmart, toy store giant in US is asked by lawmakers to provide evidence that proves the store’s toys are not produced through child labour. Palmart’s toys are transferred between 7 parties between creation and placement at the store. Palmart is considering using blockchain to solve this problem. Discretion in the reading and writing of transactions is highly important to the store chain to protect its proprietary interests. Review the following possible solutions and pick the best approach that solves Palmart’s problem:

Option A:
Use a public blockchain that supports assets.
Then create a new toy asset for each item created. 
When the toy is transferred to the next party, mandate that the associated blockchain asset to be transferred to the next party's blockchain account with required metadata.
Lawmakers can now view transactions and transfers at any time.

Option B: 
Use a permissioned blockchain.
Require that every party involved in the process to host a node on the Palmart's network. 
Create a new toy asset for each item created. 
When the toy is transferred to the next party in the process, have the previous party use a web portal to initialize a chain code Smart Contract to transfer control of the asset. 
When the next party receives the asset have them use a web portal to initialize a chain code Smart Contracts to acknowledge receipt of the asset. 
Provide read-only permissions to the lawmakers so they can receive the evidence they are looking for.