An Amateur Gardener Decides to Change the Variety of Tomatoes

An amateur gardener decides to change the variety of tomatoes for this year to see if the yield improves.He put in six plants the previous year and six plants this year,using the same part of the garden.The average yield per plant was 11.3 pounds per plant in the previous year and 14.5 pounds per plant using the new variety.What is this an example of?

A) An experiment
B) An observational study not an experiment
C) The elimination of all confounding variables by design,because the gardener used the same part of the garden in both years
D) A multistage design,because 2 years were involved