'All knew that this interest was, somehow, the cause of the war. To strengthen, perpetuate, and extend this interest, was the object for which the insurgents would rend the Union even by war; while the government claimed no right to do more than to restrict the territorial enlargement of it". - President Abraham Lincoln, second inaugural address, 1865

In the excerpt above, what does "this interest" refer to?

(10 points)
A.
The Confederate States of America
B.
Protective tariffs
C.
Slavery
D.
The U. S. Constitution