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
答案是D
但是不太明白C的解释
and even though creates a sentence fragment. 这个是为什么呢?