In this case, there is a correlation as well as a causal relationship between the amount of fertilizer applied and the crop yield. Increasing the amount of fertilizer up to a certain point, would generally causes an increase in crop yield. This is because fertilizer provides essential nutrients that the plants need, thereby, directly impacting the yield. Therefore, x$\_$n $($fertilizer$)$ directly affects y$\_$n $($crop yield$),$ establishing a causal relationship. explain further and give more contribution