Why using a subordinate here?

[GetridofG]

The Iroquois were primarily planters, although they supplemented their cultivation of maize, squash, and beans woth fishing and hunting.
Initially, I chose "but they supplemented".
Why here use "although"?

[theCEO]

The Iroquois were primarily planters, although they supplemented their cultivation of maize, squash, and beans woth fishing and hunting.
Initially, I chose "but they supplemented".
Why here use "although"?

Hi GetridofG! Could you list the choices? Often times the answer can be found through process of elimination.

[DavidG@VeritasPrep]

Initially, I chose "but they supplemented".

The answer "but they supplemented" was not an option. Take another look:

The Iroquois were primarily planters, but supplementing their cultivation of maize, squash, and beans with fishing and hunting.

(A) but supplementing
(B) and had supplemented
(C) and even though they supplemented
(D) although they supplemented
(E) but with supplementing