What was the main cause of the year without a summer?

yellow fever and other tropical diseases

seeing as no choices were provided, i'll do my best to answer it from knowledge.

job specialization promotes work ethic to the person doing the job. he/she may be strong in the arms and able to lift things, specializing him/her in a job where you lift and carry.

it allows shippers to carry commercial goods, which allows them to save time and money.