Why would be hard to mine gold from the ocean?

Because a lot of equipment is envolved and its under water. 

People would not be able to go under the water for long periods of time to mine the gold due to the lack of oxygen under the water, so instead they use machines. However the machines can only go so deep in the water before the pressure is too great and they get crushed. 

Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.