Which of the following statement is true?
(a) The process of adding blocks to a blockchain is computationally expensive, and even more so to change blocks further back in the chain.
(b) Any miner trying to add a new block of data is required to provide a cryptographic proof called Proof of Work.
(c) To produce the Proof of Work, a miner will have to perform multiple rounds of hashing with the data of the previous block such that the resultant hash value begins with a specified number of zeros.
(d) All of the above.