What was an important facet of the NATO treaty meant to strengthen the alliance?

12. What was an important facet of the NATO treaty meant to strengthen the alliance?
A. Nuclear weapons may be used if the Soviet Union overthrows any democracy.
B. Any country challenging the United States can't be a member of NATO.
C. An attack against one member would be considered an attack against all the members.
D. Member countries of NATO would have a shared currency.