A relative shortage of available farmland would create an even greater need to use land efficiently. Also, the stimulus already establishes that farmers seek to increase productivity. This makes the reduction in the use of fertilizer even more confusing.

So we're growing some new crops, but this tells us that fertilizer is just as effective on the new crops. That doesn't give us any reason why fertilizer isn’t used as much today.

The stimulus already establishes that fertilizer led to a massive increase in farmland productivity. Given that fact, why did farmers reduce their use of fertilizer in the following years? This doesn't tell us.

This gives us reason to believe that fertilizer’s effectiveness plateaus at a certain point, which explains why farmers decreased their use of it over the years. Why spend money on fertilizer that is now ineffective?

The correct answer must explain why fertilizer use declined after 1985. Factors influencing fertilizer production before that time are irrelevant, so this isn't helpful.