Teachers need to teach students how to write a conclusion based on a science investigation. When an item says to explain the answer, students must use information to support their ideas, which may come from readings, investigations, prior knowledge, etc.
Students show a tendency to fail to read the entire question.
Often they will display an excellent knowledge of the subject but they will answer a question incorrectly or insufficiently because they haven't correctly read the question or accompanying material. They need to know this and be able to write about controlled variables. Students need to be given opportunities to critically evaluate a laboratory investigation and suggest revisions to make it better. We should give more instruction on drawing conclusions. Students need to be able to refer back to information to help them support their decisions.