Which of the following is true about block mining for the Bitcoin blockchain?

Group of answer choices

a) Blocks of transactions get added to the blockchain without the transactions being verified

b) The mathematical puzzle involved in the proof-of-work (finding a "good" nonce to validate the block) is easy to solve but hard to verify

c) Every time new transactions are created, a single miner is selected at random from the community to add these transactions to a block and publish it

d) Finding a good nonce to validate (sign) the block is computationally intensive