Which of these is always part of using the scientific method

What happens whenever a net force acts on an object

How does the study of genetic disorders such as pku biologists understand normal alleles?

Ascience research group is testing a new type of plant food. one trial is conducted in a controlled experiment. the data from the trial show that the plant that received the new food grew faster than the control plant. the group announces that the new plant food makes plants grow faster. what is the main weakness in this scientific claim?