Answer:
I believe it should be:
"There is a greater demand for carpenters than dishwashers".
Explanation:
For the first option, I determined that it was incorrect because if there is a greater supply of carpenters than dishwashers, the dishwashing position would be in higher demand and would pay more.
For the second option, I determined that it was correct because if there is a demand on a job position, the pay will usually be higher since there are not many people who can do that job.
For the third option, I determined it was incorrect because both jobs have their dangers and different environments. It would not seen likely to me that this would be the best reason.
I determined the fourth option to be incorrect because, to be honest, it seemed like an answer filler. Depending on where you work or who you work for or what you make or wash, each employee is different and have different skill sets.
I hope this helps :).
I am sincerely sorry if it hasn't.
