"The word 'among' means intermingled with. A thing which is among others is intermingled with them. Commerce among the states cannot be stop at the external boundary line of each state, but may be introduced into the interior." - Chief Justice John Marshall, opinion of the court, Gibbons v. Ogden (1824)

Which of the following best summarizes this excerpt from john marshall's opinion?
a: the definition of commerce is differently interpreted by each state.
b: the most important area of involving commerce is in securing the national economy
c: The buying and selling of goods across state borders ought to fall under stricter laws
d: the transportation and navigation of goods is part of the constitutional definition of commerce