3. The amount of in the soil was no longer a limiting factor in crop production after chemists discovered how to produce ammonia artificially in 1910.
A carbon
B. chlorine
c. nitrogen
D. sulfur

C. Nitrogen

Explanation:

One way to get this answer is through context clues and knowledge of basic chemistry:

Ammonia, or NH3 is a gas that only contains nitrogen and hydrogen. Of all our answer choices, only (c.) Nitrogen is present in ammonia

Another way to solve this is by considering biology,

in which we know that plants rely on the nitrogenous wastes in soil to help with growth.