In what ways was the ocean valuable to economies in the northern colonies?

The ocean was in many ways valuable to the Northern colonies. For instance the good harbors served them well in trading and merchandising. The ships could travel carrying merchandise to Philadelphia and some other parts. As the tradition of slavery was also a booming tradition in those time, the ocean helped transport them as well, building of ships was another benefit the ocean gave them.  

#2 the answer is d

#3 the answer is a

#4 the answer is b

the answer is use of support