A study was conducted to test the effect of fertilizer and irrigation on tomato crop yield. Two types of fertilizers (Fertilizer A and Fertilizer B) and five levels of irrigation (100, 150, 200, 250, and 300 mm) were applied. For each combination of these factors, 5 replicate plots of tomatoes of the same variety were grown, making a total of 50 plots. The plots in the field were assigned randomly and independently to the combinations of treatments. When the crop matured, the yield of tomatoes in each plot was recorded in kg/m2 .

(a) Identify the study units.

(b) What is/are the explanatory variable(s) or factor(s)? State the type of data scale used (e.g., categorical, ordinal, etc.) for recording the explanatory variable(s).

(c) What is/are the response variable(s)? State the type of data scale used (e.g., categorical, ordinal, etc.) for recording the response variable(s).

(d) Is this an observational or experimental study? Very briefly explain your answer.

(e) Based on this study, will it be possible to make either population inferences or causal inferences or both? Briefly explain your answer.