Read the following​ sentence, and then select the group of words that consists of only nouns.

​Instead, he believed humans should turn their efforts to providing conditions that reinforce desired​ behavior, thus improving human behavior in​ general.
A.
​humans, efforts,​ behavior, thus
B.
​he, efforts,​ providing, conditions
C.
​Instead, humans,​ behavior, general
D.
​humans, efforts,​ conditions, general