To evaluate whether a new fertilizer improves plant growth more than the old fertilizer, the fertilizer developer gives some plants the new fertilizer and others the old fertilizer. is this an observation or an experiment? why? select the most correct answe.

a) observation: you are only watching how the plants grow and develop.
b) experiment: you imposed a treatment (new fertilizer on some plants, old on others) to determine its effect.
c) observation: you already know which plants are treated with the new fertilizer and which are treated with the old fertilizer; you're only collecting data from both groups
d) experiment: you are only watching how the plants grow and develop.